TracFone - Good for light users.

The first prepaid carrier I want to address is Tracfone wireless.  They are the largest prepaid carrier in the United States, and can be purchased at over 60,000 retailers in the United States. Phones can be purchased also at amazon.com, tracfone.com, ebay.com, and may other online retailers. 


Pros:

1.  Service is as low as $7/month.
This is great for someone who wants to carry an emergency phone to call home if they have an emergency.  I used a Tracfone for over 1 year as my 'fishing' phone.  I would throw one in my tackle box, (the $10 basic phone).  If I accidentally went for a swim, i would only be out a $10 phone, instead of a $100+ smartphone.

2.  Bonus codes for extra minutes.
That's right!  Tracfone offers free bonus minutes when you add time to your phone.  the minutes range from 10-100+ minutes depending on card added, and code used.  A good website with updated codes can be found at Turk Forums.  You will need to register, but that is free, and you will have access to many codes.  They do ask for your contribution on codes if they work or not, but it's for the good of all :).

3. Double and Triple Minutes for life of phone
Another great perk of Tracfone is that some phones come with DMFL (Double Munites For Life) or TMFL (Triple Minutes For Life).  buying one of these phones will save you money in the long run.
Some examples of this:

4.  A phone for Tracfone can be found for $10, and with a $20 card for 60 min, it's not too bad to get started with Tracfone.

Cons:
1.  Ok customer service.  Their customer service is better than it used to be.  Most of the CS is based in India or the Philippians, so they have a thick accent.  The best CS number is 305-715-6500, based in Maimi Flordia.

2. Per minutes is expensive without a DMFL or TMFL card/phone.  For a 60 Min card the cost is 33 cents a minute.  

3.  Porting your current number to/from Tracfone can be a nightmare.  I recommend using a Google Voice number and getting a fresh number with Tracfone.  

4.  You can not being your own device to Tracfone.  Tracfone locks their SIM cards to their phones so you can not use the SIM in any other phone.  Also if your SIM is deactivated, they need to send you a new one.  the SIM cards are a 1 and done type of card, never to be able to be re-activated again once used.  

Update 8.19.12  updated graph of prices 
Update 2.19.13 updated info