Chax is a very very cool free iChat plugin thingy. It gives me several much-needed enhancements to iChat, including being able to double-click an image in the chat and open it in Preview, Growl notifications, auto-accepting of text chat invites, and several "bugfixes" for iChat, including problems with how iChat handles Jabber groupchats.
The latest version includes perhaps the best enhancement of all: tabbed chats. W00t.
