I'v been using the AT&T prepaid cards from Sam's Club. Rates are always competitive, you can recharge the same card (i.e., the same access #) over and over again. The minutes do not expire, last I checked.

And, in some states, you can but a new card/recharge existing card and pay no taxes. Considering that hard wired telecomunications services are severly taxed (Feds, State, and local, ... they're running out of names to use for the the taxes levied on these services), this is a huge plus, (a protest vote).