It depends what you are looking for and when.
http://www.freeBMD.org.uk has the index for births, marriages and deaths from 1837 - c1914.
http://www.familysearch.org has the 1881 census index, plus the IGI (transcriptions of mainly births and marriages from 1500's to around mid 1800's, some deaths)
http://www.rootschat.co.uk has message boards for all sorts of different things.
http://www.genesreconnected.co.uk is very good but you have to join the site in order to use the message boards, you can look at them for free but can't add to them.
http://www.cyndislist.com has links to thousands of other family history websites.
http://www.ancestry.co.uk whilst this is a subscription site (and worth every penny) you can register for free and get the 1881 census index plus the full BMD indexes for 1837-c2004.
Hope this helps