"Zoopla.co.uk is a unique property website offering users information and tools to help them make better-informed property decisions. Our aim is to provide the most comprehensive source of residential property market information in the UK to help buyers, sellers, owners and estate agents alike and give them an advantage in the property market." Says the official blurb.
It's also a bloody good way to have a nose at how much money you've made on your house since you've moved in (and sadly if you move in recently how much you've lost)
For me it's a bit like google maps, and live maps. I aren't planning on moving, but I am a little bit nosy so I find it's interesting to see how other houses (especially the same type of house as yours) is valued on the site.
It's a little hit and miss, and you can refine your own house to give the estimators more data to base your house price.
Worth spending 10 minutes having a nose around.