It's free to register (they even give you a $10 sign up bonus if you make one purchase by 12/31/08), and the cash is automatic ; no pesky mail in rebate forms. Plus, this cash back is in addition to any incentives tied to your credit card, such as airline or hotel miles. The only downside is you must wait a few months from the date of purchase to receive the cash.
You continue to shop at the same websites you normally would, except you click through to them via the Ebates website. This activates the cash back monitor. Once you make the purchase, Ebates is alerted and you receive notification of a pending cash payout.
I use Ebates before making any online purchase - travel or otherwise. Over the past 14 months I have received $180 cash back via Ebates - enough for a free airline ticket!