
I'm tracing mine. I started here
http://www.genesreunited.co.uk/This is a good site because if you put your tree on here and names match other members' names the site will match you up and you then have the option of contacting that person or not. The contact is through the site's system so the other members never actually have access to your personal email. There is a forum where some very experienced members hang out and they are more than happy to answer queries.
The only drawback with this site is that you have to have some success already to get anywhere. This is because until you get to the 1800s you are unlikely to find people searching the same lines. Most people, including myself will not allow free access to any family names withinthe past 100 yrs in order to safeguard the privacy of living family members.
I do have access to another site
http://www.ancestry.com/ which costs about £60-70 per year's subscription. This gives access to all the Birth Marriage Death records going back to when they first became officially collected about 1837 and also the 10 year census records going back to 1841. These are fabulous because you can see the original handwriting etc.
The site I use is great but not perfect and there have been some transcription errors that lead to finding families tricky sometimes. Also people used to lie, change their names, ages etc. and sometimes they disappear hmm? Australia

all very fascinating stuff and the more you get into the more you find there is quite a lot you can do online i.e. there are sites that allow access to transported criminals sent to Australia.
If you get back as far as the 1841 census it is very likely you are completely addicted. This is when you can start checking out the Mormon church site
http://www.familysearch.org/Eng/Search/frameset_search.asp?PAGE=igi/search_IGI.asp&clear_form=true They have transcribed quite a lot of parish records, some going back to the 1600s and probably earlier. These are not comprehensive though and there are mistakes that need to be checked out and that is the stage where you start looking to go to the parish records yourself. That's the point I'm at now and have already been to the London Metropolitan Archives to search for one of my London lines but I have to tell you this is labourious, frustrating, disappointing and thrilling but more than anything it is extremely time consuming. From what I know now if I want to trace further back for my other family lines I will have to go to Surrey, Sussex, Bristol, Somerset, Yorkshire, Ireland and Russia - well perhaps not Russia :D
When I started I thought it would be a short term (few months) pursuit but I now realise this is a lifelong hobby.
I now have more than 70 grandparents' names and yet I still haven't even managed to get started on one of my great grandfathers' family yet. With the help of other people on GenesR I have managed to get back to the early 1700's on one line but that's the earliest. To be honest I am now getting more interested in the social history that the families were living in at the times and places they lived as this explains their movements/migrations. There is a site that shows descriptions of London City streets by Charles Booth. Very enlightening, giving a chilling insight to the conditions some people were living in during the turn of the century.