Phone Power: Works well and cheap
I've been referring them and getting the $10 referral credits for a while now. (Just have to give the other person your username or phone number) I do pay monthly instead of prepay so that brings my bill to under $10 some months. They've changed the website a bit from when I signed up and I feel like its a little harder to navigate now, but they have also added more features. Customer service is actually in the US so if you call they are pretty helpful.