They are nice black plastic credit card sized gift cards with a unique promo code for a free $20 UBER credit to new users.
I figured I'd try one to see what it gives me so I just signed up for an UBER account (requires a mobile number and credit card) and entered one of the codes and it says it gives me a free $20 UBER credit for a ride in the San Diego area and expires in December 2013.
Anyhow, looks like I've got 20 of them that should be good to use if anyone is interested.
Redeem at UBER.com/promo
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Figured I'd add the codes to the Wiki here... just delete them once you've used it...
21B940849 <-- used by newrui
21B684082 <-- used by jcknight07
21B628978 <-- used by smiless