Great for trying to figure out what to do with your friends on any given evening. You make "groups," which are basically threads where everyone can join in on the textversation. Each group even gets it's own phone number that everyone can call to talk at once.
I've started using GroupMe recently, and it's definitely useful for group messaging. I also like that you can switch between data / SMS use quickly and easily (and it even offers to help you do so when you're lacking one or the other).