I've been using Ebates for ages--1999? 2000? I've earned around $400 in dribs and drabs. You get a percentages of online purchases back as a rebate (Ebate) by shopping through their links. Like MyPoints, you earn something for activity you do everyday. Unlike MyPoints, you get straight cash with Ebates, instead of points that you exchange for gift cards.
Ebates is having a no-brainer, simple promotion right now: sign up, make 1 purchase through Ebates, and get a $10 bonus ($5 cash, $10 Target gift card--your choice). Learn more here: Ebates $10 promotion.
In the interest of full disclosure: I earn a gift card when you sign up through this link. Such is the Way of the Internet Kickback.