When I bought my iPhone 3GS, one of the helpful Apple retail guys told me to give textPlus a try for sending text messages, as I'd bought into the cheapest texting plan AT&T offers. I've since graduated to unlimited texting, and had left textPlus by the wayside until I found out it had a blossoming community angle to the service.
While you can still send free texts until your fingers fall off, the latest version of textPlus (an ad-supported app, although there's a paid version as well) allows you to create, participate and join in Communities, which are much like topical group chat rooms seen on AIM back in the early days of the service.
Once you've set up a basic account using not much more than your name, zip code and birthdate, textPlus looks a lot like a big chat application. You have access to your own Address Book and can send a simple text to anyone with a mobile number and the ability to receive texts. They can text you right back, and it'll appear in your Conversations panel.
This is an easy way to see basic conversations, and if you're texting from a "dumb" phone you can still use the service as a simple conduit. This works one-on-one and in groups, although there are various commands for simple text input that'll allow you to traverse group conversations and name yourself, etc.
Read on for more...