A good place to start is Martins site
http://www.moneysavingexpert.com/shopping/cashback-websites
Just about anything you buy on the internet can be bought via a cashback website, the cashback site gets paid commission for directing the business to them, these cashcack sites then give you a cut of the commission. you can even earn without spending like doing a Gocompare Car Insurance search, I got £1.50 for doing that recently.
Just a few examples of my cashback in past few months
1st colomn is company I bought from, 2nd how much I spent with them 3rd cashback amount (1 point = 1p)
4th date of transaction.
As you can see with my car insurance £70 cashback on a £213 policy is a high %.
Aviva Motor Insurance £213.00 7000 points 17 Oct 09
Simply Business £236.08 2400 points 13 Dec 09
netflights £1274.01 1274 points 09 Dec 09
Walt Disney Travel... £2747.00 12362 points 03 Jan 10
4 transactions, £230.36 cashback.
Been doing it for 5 years now, claimed £1,932