Virgin Mobile. Its what I have. Its owned by Sprint so if you have Sprint coverage in your area youll have service.
No contract, lowest plan is $25 a month. All have unlimited text+web.
25 Dollars - 300 Calling Minutes
40 Dollars -1200 Calling Minutes
60 Dollars - Unlimited Everything
If you need to get a phone now, Id recommend the LG Optimus V. Do not get the Intercept it sucks ass. I have the Optimus V now and its a really nice phone.
However if you can wait, this summer theyre going to be releasing the Motorola Triumph, with a 1Ghz processor and 5MP camera, along with a VGA front facing camera and it has Flash (as in Adobe Flash). This is what Ill be upgrading too.
Both are 3G
LG Optimus V

Motorola Triumph

Site:
http://www.virginmobileusa.com/Oh btw, Sprint just signed a contract very recently with Motorola so VM is expecting around 10 new Motorola devices by the end of the year