http://www.nethouseprices.com/

This site lets you see the prices that individual houses and flats sold for (using data from the Land Registry). You need to register before you can see the prices.

At the moment it is completely free, they are going to start charging for recent sales soon.

Very helpful for working out how much your house is/may be worth.

Looks like my house has gone up £100k in four years. I'm rich, I'm rich, I'm rich...

...of course every other house in the area has gone up the same.