There are some very good paid instant messaging apps for the App Store, and plenty of free options too. Meebo is the first that manages to be both, with almost no compromise.
Meebo was one of the first companies out of the gates with an instant message web app for the iPhone, so it's weird that it took them this long to go native. But hey, whatever, they did, and it's goddamn fantastic. Here's the cream of the feature list: