I have registered for the site, which allows you to use the tree-making tools for free. I can do basic searches, but cannot access the results without subscribing. Like BB, I've used FreeBMD, and I've used FreeCen, however because these are voluntary projects, the data available can be limited.
Some parish records are also available online for free; I've managed to find about 100 years of family history from one such resource. If your family are local, there may also be information in the local library; I found out a lot of information in Ratby just by visiting the library there one morning.
I would like to subscribe to Ancestry.co.uk for more information, but I don't have the time to commit to using it at the moment. As mentioned above, it can be addictive!