We've been using Quidco for a couple of years now. It's a no-brainer really. For day-to-day internet purchases, you simply log in, find the merchant you want to use (almost all the significant online retailers are on there), follow the link then from that point you shop as normal. If we shopped online for groceries etc, it would be even better for us, but for stuff like car insurance, recovery, home insurance, Sky TV etc, its fantastic.
I got about £40 cashback on my Aviva car insurance policy, £65 cashback on my Sky HD subscription etc etc. You do have to wait and sometimes they don't track properly, or default etc, but generally it's all good. We've had about £500 since we joined.