I use MyPoints, because I get HiltonHonors points. I order office supplies for my job and use the Office Depot link where you get 5000 points for every $400 order. For us that's a couple of flip chart pads, toner cartridges and cases of paper. They have other levels too $100, $200 etc. The MyPoints are 1000=1000 for HiltonHonors points, so after a few months you're eligible for a room. I haven't cashed in any Hilton points yet, but the points transfer over pretty fast from MyPoints to Hilton.
I've also used e-rewards (took about a year for first batch of Hilton points) and GoZing.com. GoZing.com pays in cash, but you can get disqualified from a survey by using the wrong browser with no chance to switch browsers if you can. I haven't qualified for a survey in about a year.
Not a membership, but I sold a ton of old college textbooks on Amazon.com earlier this year, some more than 3 years old!
Have fun!