I've never seen a card that gave rewards for truly free balance transfers. Watch out for those "0%" (but with a 3% transaction fee) cards.

The best BT card I know of is the Chase Cashbuilder. It has no annual fee and 0% + no transaction fees on BTs for a full year. They seem generous with credit lines, and even give you balance xfer checks so you can write yourself a check and deposit it for the full amount of your credit line. Three catches:

1. Watch out for missed payments, including "universal default" (see BigLar's post above). All it takes is one missed payment, and you're screwed.
2. Be aware that maxing out the limit will have a temporary negative effect on your credit rating. Once you pay it off, your credit will be good again within a short time.
3. It's not 0% for purchases, so do NOT use it for purchases or you'll have to pay off the full BT amount before you can pay off any of the purchase amount.