Kajeet has a $14.99/month plan (no contract - pay monthly as you go) that is unlimited text and 60 phone minutes. You do have to buy their phones though, that is where they get you. Phones start at $24.99 for the basic flip phone & go up to almost $300 for the Rumor LG Touch. They run on the Sprint service. But if you just need a basic phone & only use text, this is a great deal.
straighttalk.com - uses the Verizon network, $45 per month prepaid, unlimited talk, text and internet. You can buy the phones at Wal Mart but their phones online are cheaper (FREE!) and include a month of service.
Just text you can get T-mobile for $15 a month but I prefer Virgin for $25 300 min, unlimited web & text. Kyocera loft can be found for as little as $39 & I like it. It's like a baby blackberry.
A friend of mine has Virgin Mobile. They have unlimited text, 300 phone minutes and unlimited data for $25 a month. They love it and pay $100 a month for 4 phones. We have four phones with AT&T and pay over $200
I will chime in on Virgin Mobile. We use it and my wife has the Kyocera Loft and for $25 a month you get 300 mins and unlimited text and web. No issues at all and it is pay as you go.