http://www.quidco.comYou buy stuff online and most of the commission goes to you, the site takes a small admin fee.
I bought my house insurance last year for £138 and got £120 cashback, so it only cost me £18.
I got my cheap broadband too, and got £28.
There are a lot of well known companies on there, you can also get cashback when you buy stuff on ebay. (I think it's only 5% of the original listing fee, so is pennies, but they soon add up).
Some merchants take a long time to pay out, the house insurance took 6 months, as they need to see that you are making regular payments for a while to make sure you are a genuine customer and not somebody out to scam.
It's best to clear out your cookies before you login to the site, otherwise the commission could go to somewhere else, as it is based on cookies being there.
If there's a cookie from another site (e.g. google or msn) then the commission will go to them.