I've used Upromise for about a year and a half now, and have earned $333 and change in that time by registering my cards, and using the Upromise toolbar. It won't get my student loans paid off overnight, but every little bit helps.

In any case, they'll send you a check now if you'd rather have that.

More info at http://www.upromise.com/welcome/how-it-works/redemption