eBuddy has long been my go-to IM app for Android because I think it looks and works the best. However! The new imo.im app (which is available in beta right now) has gotten quite attractive and is a huge improvement over the current official version.
The beta uses a tabbed interface (sorta similar to eBuddy) and has a brand spanking new UI where everything is a lot easier to use. The beta is only a "preview" of what's to come in the official 2.0 update but from what I'm seeing, it's pretty much ready to go. One thing: I don't like how the "Send" button is near the text box and not on the keyboard itself. [Android Market via Imo]