Heysan is a free chat client for MSN, AIM, ICQ, Yahoo and GTalk. It uses a web interface so you don’t have to download software on to your phone.
It works very well.
It also sends auto sends a message that you are using Heysan to anyone who IMs you and has advertising. It may or may not be a reasonable trade off depending on your needs. Also away messages are not supported at this time.
The mobile interface for the site can be found m.heysan.com.
It’s probably worth a bookmark and will do in a pinch, I’m not sure you want it for your main IM client.