I use and recommend Net 10 for pre-paid service. It is way cheaper than Tracfone.
300 minutes for $30
600 minutes for $60
*and just this past week they introduced their lowest minute card:
150 minutes for $15
The phones can be purchased at Walmart, Kmart and Target. I have the Motorola c139, it goes for $49.99 comes with 300 minutes of airtime.
NET10 Wireless - Pay As You Go Made Simple