I've been using Ebates for just over 2 years, and it is amazing. Like getting free money, it's safe, and it's super easy to use:
1. Sign up for an account
2. Use their links to go to the website of your choice
3. Complete your purchase using the same browser tab.
4. The rebate will automatically be deposited in your Ebates account.
To collect your payment, you can either have a cheque mailed directly to you or you can have it deposited to your Paypal account. Payouts are processed every 3 months. You don't have to give them any private info (except your email) so you can feel super safe.
I made about $80 last holiday season in rebates and that's on items I would have purchased anyway. Win-win. The rebates are usually 2%-5% but I have seen some go up to 20% (at Old Navy!) On the US site, you get a $10 bonus after you spend $25 on your first purchase!
I'm located in Canada, so I use Ebates.ca. If you are in the US, you would use Ebates.com