I use easyfundraising and they even have a nifty toolbar addon that tells you when a website you're on is included in the donation scheme. I use it for Small Pet & Animal Care, which is a small Hull based charity that's seriously struggling for foster homes and fundraising. They've just taken on a mother with a 2 litters (9 cats total) and it's stretching their coffers somewhat).
Highly recommended for whichever charity is close to your heart.