iPhone/iPod touch: We've been fans of popular chat app Meebo for a while, so it's great to hear they've released a version for the iPhone that competes with premium chat apps in the App Store but doesn't cost a dime.
Meebo sports push notifications that activate automatically when the app is closed so you can keep in touch even when Meebo isn't running, and it syncs between your iPhone and desktop app so you don't miss a beat communicating with your contacts. It's got all the features its web-based cousin has—like full chat history and integration with Facebook, AIM, and Yahoo.
Landscape mode and other little niceties like the ability to add Away messages make using Meebo on the iPhone a joy. It doesn't have a whole bunch of useless features weighing it down so the learning curve on this app takes about 10 seconds.
If you're not rocking an iPhone, Meebo hasn't forgotten about you. The service already has apps for Android (and a mobile web site for other smartphones).
Now that instant messaging apps are finding their way off the desktop and onto mobile devices, what features would you like to see in future versions. Dream big in the comments.