I was researching which credit cards to use to get the most rebates (this is assuming you have good credit and don't carry a balance), and here are my results:
Use a AAA Visa card (only available to AAA members) for gas purchases
5% cash rebate at any gas station, no annual fee
Use AmEx Blue everywhere AmEx is accepted
Up to 3% (0.50% for $0-2K, 1.00% for $2-6K, 3.00% for $6K+) rebate at supermarkets & home improvement stores;
up to 1.5% (0.25% for $0-2K, 0.50% for $2-6K, 1.50% for $6K+) rebate elsewhere
No annual fee, get $20 if you sign up at moneybb.com (you can mention my referrer name - dolginov)
Use Fleet Cash Rebate everywhere AmEx is not accepted
1% non-tiered cash rebate, no annual fee
There are also frequent flyer miles cards, but most of them have an annual fee, and miles are just not the same as the cold hard cash. Delta AmEx, in particular, comes to mind, which gives 15K miles on sign up + a free US companion air ticket for a $135 annual fee.