This is my choice of websites which in my opinion every student of military history should bookmark. To merit inclusion they had to merit both scholarship and be genuinely useful to researchers. I decided not to include any sites about individual's experiences, individual types of equipment or with the exception of Wikipedia general historical sites. They are in no particular order but I hope include something about each of the services and a variety of historical periods.

Wikipedia
The internet has produced a large number of sites where scholars and enthusiasts from around the world work together to produce something of greater value than any one individual could by themselves. The best example of this collaboration the wiki (apparently from an Hawaiian word wiki wiki meaning fast) which has been defined as a website that allows visitors to add, remove, edit and change content, typically without the need for registration.
The most important of these is Wikipedia which has some 1.7m entries in English (and hundreds of thousands of more in other languages as well). Wikipedia is often criticised for its inaccuracy and bias, but in my experience the military history articles are spot on.
And in case if you spot a mistake you can always make a correction.
http://en.wikipedia.org/wiki/Main_Page

Commonwealth War Graves Commission
Since 1917 the Commonwealth War Graves Commission has been caring for war graves of men from Britain and the Commonwealth who died during the two world wars. They maintain 23,000 cemeteries across the world. Visiting any of them is a profoundly moving experience, as their calm and immaculately kept grounds bring home the horrors and futility of war.
Their website is based on the Debt of Honour Register which records details of the 1,750,000 men (and a few women) for whose graves they care, together with 67,000 civilians who lost their lives as the result of enemy action during the Second World War. For each individual there are details of his unit, when he died and where he is buried. Often there may well be details of next of kin and the inscription which is on the gravestone. The register will tell which in which cemetery he is buried (with plot number) and how to get there. There is even a brief history of the cemetery itself.
There are also pages about the history of the Commission and material for teachers.
www.cwgc.org

Land Forces of Britain, the Empire and the Commonwealth
This is one of the oldest sites on the internet having been launched as long ago as 1995. The aim was simple to provide brief details about every Army unit which served under the British crown. This was an ambitious project and inevitably some units are pretty well covered, while there is very little for others. One of the great strengths is that is simply laid out and easy to navigate.
Each page should contain information such as title changes and lineage, battalion service histories, battle honours, badges, lists of colonels, alliances, bibliographies, etc. as well as links to "every" known regimental page on the internet (official, historical, biographical, re-enactment, etc.).
Several features are cross-referenced to other sections of the site. For example, the battle honours for each regiment are linked to war pages which give more information about the context of the battles and their wars. The site is designed to help you trace the changes in designations of regiments, and the complexities of their lineages. As of 2004 there are well over 2,000 such pages.
Unfortunately the site is still very incomplete and does not seem to have been updated since mid-2005. Even so it is the first port of call if you want to find out basic facts about a British (or Canadian or Australian, etc) Army unit. NOTE this site was down in November 2008 and had been for some months.
www.regiments.org

The Australian War Memorial
Despite the name the AWM, in the heart of Canberra, is one of the world's great museums. Even non-military enthusiasts can easily spend a day there and not see everything (the website has a virtual tour so you can what you are missing).
Like London's Imperial War Museum it was founded to commemorate the sacrifice made by thousands of young Aussies during the First World War. As a result the emphasis is very much on the people rather than the kit they used or even the battles they took part in.
In recent years the Memorial has placed a large number of databases online, ranging from a Roll of Honour to Australia's 100,000 men who died in wars since 1900; nominal rolls for men who served overseas (some actually provided by the Department of Veterans' Affairs); details of awards and honours and some information about prisoners of war.
Indeed if you are researching individuals who served in the Australian armed forces (the vast majority of whom had close family links to Britain) until the late 1960s you should find them here.
The only major exception is service records, which are now available online (at least for the First World War) from the National Archives at www.naa.gov.au. There are also war diaries for Australian units, examples of artefacts from the collections, and a number of special exhibitions. I found the one on colour photography between 1914 and 1918 particularly interesting.
There are even several blogs, but when at the time of writing they were not operational.
http://www.awm.gov.au/

The Asplin Military History Resources

Fortunately the internet has room for both the highly professional websites posted by archives and museums, and those which are quite clearly works of love from one individual or small group, and which can put the sites of much larger bodies to shame.
Chief among the later is this website devoted to the later Victorian and Edwardian Army. By profession Kevin Asplin is an Army officer and medal researcher. His main interest, at least initially, seems to be the Imperial Yeomanry, mounted troops who were recruited during the Boer War. There is a list of all 38,000 men who served in with the Yeomanry. Mr Asplin has been adding other resources, such as lists of men who joined colonial units during the Boer War; medal rolls for various colonial conflicts, casualty rolls for the Crimean and other wars of the period; some records for individual regiments (you will need to check to see whether yours is here) as well as a few individual soldiers; and a page describing the major sources for military research at Kew.

The old URL no longer works but he has a new website at www.britishmedals.us/Kevin where he has taken the opportunity to lay the site in a more logical way.

The National Archives

You will need to visit The National Archives at Kew if you are researching almost any aspect of British military history from a soldier's career to the development of post-war missile systems.
The breadth and range of their holdings is reflected on the website, which frankly can at times be confusing to navigation.
Highlights include a range of research guides explaining what records they and how you can use them: inevitably most are aimed at family historians, but there are a few more specialist titles. They link to the online catalogue which has descriptions of over nine million documents, maps, files and parchment rolls. In some cases the catalogue contains more detailed information - for example there is a list of all men for whom there are soldiers' documents between 1760 and 1854.
There is a new Your Archives 'wiki' facility at yourarchives.nationalarchives.gov.uk whereby you can add information about a particular record or series of records.
In fact the site actually contains at least three different catalogues. You can search them altogether (using the grandly names "Global Search" engine) by using the search button on the home page.
In addition, you can order documents in advance, there are a number of online exhibitions some on a military theme and the Documents Online service where you can download scans of particular documents generally for a fee - most of the records here are military..
http://www.nationalarchives.gov.uk/

Britain's Small Wars
From the insect ridden jungles of Malaya to the frozen hills of Korea, in the back streets of Aden and Cyprus, from the African bush of Kenya to the rain soaked hills of the Falkland Islands, young National Servicemen and regular British soldiers have been defending the British interests and her allies for six decades now. Their story is told in this extraordinary website.
It is extraordinary because as well as the usual (and informative) pages about the kit and the engagements fought it features at times extremely honest and always well written articles and comments by those who were there, both officers and other ranks, which descends the what the nostalgia for a lost youth one can find on lesser sites.
Occasionally one gets a feeling that they may indeed be revealing things which perhaps still officially remain secret. This is particularly true for conflicts from the 1960s onwards.
This is excellent design and easy navigation.
http://www.britains-smallwars.com/ 

Naval Portmanteau sites
For some reason there several excellent general websites on naval history. It is something which does not seem to be happen either for the Army or the Air Force, where most sites tend to concentrate on a particular topic. I have chosen five as a top 10 website choice (it was my book after all…)
They all do different things, although there some overlap and if you are studying the Royal Navy particularly in the 20th century you will need to visit several of them.
The website of HMS Surprise http://home.wxs.nl/~pdavis/index.htm was originally a site about William Loney, a 19th century naval surgeon, but has developed into a superb resource about the late 18th and early 19th century RN. You can even download software allowing you to 'navigate' HMS Surprise a navy 'frigate' of the period, best known from the pages of Patrick O'Brian's novels - fascinating.
www.cronab.demon.co.uk offers a superb resource about the Royal Navy between the eighteenth and twentieth centuries, although it is pot luck whether you will find what you are looking for. Even if you don't you are likely to be diverted into something more interesting. Unfortunately it is hard to navigate around (you need to keep scrolling down the home page) and is very text heavy. Unfortunately it seems to be 'down' when I checked in late November 2008.
If you are researching the Navy in the 20th century then Gordon Smith's Naval History Net should definitely be visited at www.naval-history.net. There are pages about all aspects of the Royal Navy during, and between the two world wars and the Falklands Campaign. It is at is best for the Second World War.
There is a sister site www.worldwar1atsea.net for the First World War period.
The next two are about aspects of the Second World War.
http://uboat.net started life devoted to the U-boat campaign with details about all the German submarines, their captains and their fates. But it has broadened out into an excellent site about the Battle of the Atlantic and the Allied response to the U-boat threat.
Meanwhile www.fleetairarmarchive.net has a mass of material about the Fleet Air Arm, from detailed descriptions of the aircraft flown, histories of FAA Squadrons and rolls of honour for the men, as well as pages about the carriers they flew from.

Conscript heroes
The book has dozens, may be hundreds, of websites devoted to the exploits and the memoirs of the famous, the infamous and the obscure. Many of the sites are excellent testimonies to the horrors and the humour of war. And it is only appropriate that at least one such site should be in my top ten choice, but which? After consideration I have chosen the website devoted to Peter Janes, who evaded capture by the Germans in 1940, later made his way along the Pat O'Leary escape line to freedom in Spain. It is a fascinating story well told by Janes' son with a lot about escape and evasion in France in general.
The site is well designed and well laid out with lots of illustrations, although inevitably there are few from the wartime period itself. You get a real feeling for the man and his experiences.
www.conscript-heroes.com

The Battle of Britain
For some reason there is a crop of excellent websites about the Battle of Britain, each of which is attractively designed and full of fascinating material. My favourite, partly because it clearly is a labour of love, is www.the-battle-of-britain.co.uk. It has lists of the squadrons and brief biographies of many of "the few" and technical descriptions of the planes.
In addition there is a day by day diary, and, chillingly, a speech made by Reichsminister Richard Darré reprinted here. If you want to know why the Battle had to be won, his speech is evidence enough!

Buy Military History on the Internet so, for once, I can get a round of drinks in for my friends.  And if you have other sites you like to recommend (or include in a new edition) let me know.