There's one more site I've been using recently to save a few more pennies and that's www.quidco.com. This is a cashback site with ties to hundreds of online shopping sites such as play.com, Tesco, Dell, HMV, Game etc.
It's free to sign up but they take a £5 subscription from your earnings annually. So if you don't use it there's nothing to pay.
The concept is very simple, create an account and login to Quidco. From the home page find your online vendor and visit their site from the link on the Quidco site. This is the important part and the link you use contains referrer information used to attribute your spending to your account.
Quidco pass 100% of the earnings to you and this might be anywhere from 1 to 10%. eg Play.com - 6% on clothing and accessories and 4% on everything else.
Some retailers pay Quidco quite quickly, others take months and the status of each retailer is clearly displayed on the site. There's also a handy earnings page where you can see what you earnt and where. Any monies you earn are paid to you by PayPal or BACs. I've been signed up since July 2007 and don't use it that much but have already earnt £45 (£40 to me £5 to Quidco). All for just doing what I would do anyway.
More info here.