Although it's kind of cheating, you could use TwinVNC to bring up a Linux desktop on an Intuition screen and use some Linux IM client. I run a couple of TwinVNC instances on my A1200 so I can browse the web with Opera on one screen and have movies playing on the other. For chatting it's very usable (not so much for videos but still fun to watch and drag screens down over each other while it's all running).
