Today has become Facebook day, whether we like it or not, it seems. If you're sick of the social network, but still like the idea of having a floating app to message your friends, Ninja SMS may be what you need. This app commandeers your text messages when you receive them, hovers over whatever you're doing, and lets you roll up the conversation when you're not in the midst of a message.
Curiously, there doesn't seem to be a way to initiate a text conversation with the app. You can only wait for someone to message you first.