Now for my first post the directly applies to only one carrier: Simple Freedom.
Simple Freedom is, well, pretty simple. You pay 25 cents per minute for calls in a lot of the US and 50 cents per minute for calls from almost all the rest. Text messages are 8 cents to send or receive. They have one color phone that they sell at Wal-Mart for $50 and a black and white whone they sell at K-Mart and Sam's Club. There are three different values of cards that expire in sixty days ($20\80 minutes local, $30\120 minutes, $50\200 minutes). There is also a new $100 (400 minute) card that expires in one year. They used to have $25\90 day and $55\120 day cards at Sam's Club but now the $100 card is here so I guess they've discontinued them. Simple Freedom also sells monophonic ringtones for about $1 plus SMS costs.
The rates are on the high side but so is the coverage. You will most likely be in an area that is local and so you'll get the 25 cent rate all the time, since the local area covers at least 2\3 of the US. The roaming area is expensive, but no more expensive than other companies' roaming (if they have any) and takes the coverage up to that of Verizon and Alltel combined (Simple Freedom is run by Alltel, with a very good Verizon roaming agreement). Text messaging also may or may not work. In roaming you have to double-dial but this isn't too bad as all you have to do is just redial the number you want to call. Customer service calls, as well as 911, are free.
Their phone is a solid phone with good reception, and can roam on analog. But it is pretty much their only phone available. As said before, they only offer mono ringtones and texting is unreliable so you're mostly limited to voice, Caller ID and similar voice-related digital services.
Overall, I'd say Simple Freedom is the best choice for a lot of people, with very strong coverage and easy-to-figure-out rates. However, high per-minute prices (25 cents at least), unreliable text messaging, and a very limited phone selection make it a second choice for heavier, more variety-conscious prepaid users like me, as does the lack of local numbers in some places they have phones. But then again, their customer service will tell you that, so I give them a nice bonus there, and I'm pretty sure you can port in. Their (fairly small but comprehensive and simple) website is at: