Updated: Best Buy (and now Amazon) are currently rolling back the price of a Cricket Wireless SIM card. For a limited time you can pick one up for $1.00 instead of the regular price of $9.99. This deal is worth noting because if you were to buy a SIM card directly from Cricket Wireless you'd also have to pay $9.99.
Cricket Wireless is an ATT owned MVNO that offers a good price to feature ratio. A summary of the MVNO's prepaid wireless plans is below.