Probably one of our favorites for chat programs on the iPhone, Heysan lets you sign in on three different popular networks. You'll be able to log in to MSN, AIM, and ICQ for instant access to your buddy lists and regular chat commands. Yahoo and GTalk are coming soon, according to the developer's Web site.
iPhone link: http://m.heysan.com/
Web site link: http://heysan.com/