This is a completely legit company owned by American Express. This is their answer to Paypal. And, since this is a new concept, they are promoting it like crazy!
Go here to sign up for a Serve account and you'll get a $10 credit.
Then, go here and play the game (as many times as you need to) to get another $10! (Thanks, Hip2Save!)
We signed up for Serve a couple of days ago, and already got our debit card in the mail (see above!). Best of all, the account already has $20 in it! For nothing! :)