Ancestry: I have used this site a great deal in my research. I pay about £70 a year and have access to all the UK censuses along with GRO indices and a lot of other stuff all of which can be downloaded at reasonable quality. I must have a hundred or more census returns saved to my hard drive and linked into my tree with Family Historian - all from Ancestry. OK it's not perfect, there are occasional errors in transcription of the census data, not all BMDs are yet searchable and the searches could be a bit more flexible. However if you want a one stop shop with at least a 90% chance of being able to trace the census entry or BMD index you want I don't think it can be beaten. For me it has been great value for money. It is easy to upload a 'gedcom' file to the site, they are improving things all the time, with the ability to add pictures and stories now if you want to. Their display of the data in ancestor tree format is very impressive. Surprisingly I have few contacts through this site despite having had my tree on it for the best part part of twelve months?

Genes Reunited: Most of the contacts I have made with other researchers interested in the same ancestry as me have been contacted via this site. For me its strengths are that it is a British site and many people join in an effort to trace new  family members. It is inexpensive, about £5 a year I think and they now provide an excellent view in descendant format of your tree which can be uploaded as a gedcom file, pictures can now be included. In contrast to Ancestry, where you can make your tree 'public' so anyone can view it, on Genes Reunited you have to proactively provide access on an individual basis. Some may feel this is a bit restrictive others will like the control and security it provides. A big plus for GRU is the way it manages contacts for you, I find this particularly helpful.

General Register Office: The online ordering site for BMD certificates, they cost £7 each for normal delivery which in my experience is reliably within a couple of weeks. Overall an excellent service which i have no hesitation in recommending

FreeBMD: A great site which I believe led the way in making UK BMD records, census records (FreeCEN) and parish records (FreeREG) available free on the web. They seem to have some sort of deal with Ancestry these days as their information is now available on the Ancestry pages. This is convenient if you use Ancestry regularly (saves clicking through to a different site), however there are occasions  when the BMD search provides some advantages and FreeBMD also provide excellent information on which records have been transcribed and whether or not they have been verified - and excellent set of sites all round.

Family Search: This is the Mormon site based in Utah USA. I'm not sure, but I believe that these people were amongst the leaders in making genealogical data availatext-align: left;ble on the web. It is a free to use site and has a global perspective. It covers a huge range of data, they claim the largest collection of free family history information in the world. Certainly a search on this site can often throw up a confusing number of possible hits and it is worth taking time to work out how to narrow down your original list of possibles. The most useful database they maintain is the International Genealogical Index (IGI) which  is a huge collection of parish record indices including the largest collection available of British parish records from around 1500. However there are some caveats, this is an index and you really do need to check the original parish record, there is much duplication and there are big gaps. For more on the IGI try the Familyhistory UK site. Personally I find this site most helpful when I am looking for information that is pre the start date for UK BMD or Census records.

Find my past: Occasionally find this useful when I can't find a specific census entry on Ancestry. The search provided here can sometimes turn up the record you're looking for. It is a pay by subscription or pay per view site. Recently has added passenger lists to its databases which are expanding all the time. 

National Archives:  This is an great site to either enable you to find where the record you are looking for is archived or  to download copies of selected records for a fee of less than £5. I have used the 'documents online' section several times to download copies of wills and details of World War One Medals. I was also able to discover where records of the Hanwell Lunatic Asylum from around 1870 were available to view, this led to me sourcing admission records and the post mortem report of my 3x great grandfather Thomas Edgeller - maybe I'd have been better not knowing!! Altogether an excellent site that is growing by the day.

Historical Directories: A fascinating site produced by Leicester University. They have developed a digital library of trade directories for England & Wales between 1750 and 1919. Some of these make interesting reading and the occasional nugget of information about one of your ancestors can be turned up. I don't find it a particularly easy site to use but it generally repays the effort of learning how to search and navigate.

GENUKI: A great collection of genealogical information for the UK & Ireland. I have mainly used its pages when trying to sort out which parish records I should look in for the records I want. However I'm certain there is much more I could get from this site if only I remembered to visit it more often!

Find a Church: I have used this site on odd occasions particularly when looking for pictures or location details of a particular church - it's quick & simple to use

Internment.net: An ever growing resource providing information on names and places of burial. Well worth a visit now and then, there is a quick search facility which can turn up the odd nugget!

Commonwealth War Graves Commission:  Established by Royal Charter in 1917, the Commonwealth War Graves Commission pays tribute to the 1,700,000 men and women of the Commonwealth forces who died in the two world wars. It is a non-profit-making organisation that was founded by Sir Fabian Ware. 

Archaic medical terms: A great site if you can't make head nor tail of a death certificate for example - particularly useful when reading old autopsy reports - see above!!

Historical word meanings: This is an Oxford University Press site I have used now and then to decipher the meaning of various old documents. Whilst its purpose is not specifically focused on ancient meanings I have usually found what I was looking for.

The Workhouse: I just like wandering around this site!

Ships and their names: I have been able to trace pictures of a couple of the ships some of my ancestors emigrated on here - useful in conjunction with the passenger lists mentioned above.