Adium is the major player in alternative IM for that platform. Other similar alternative IM clients, such as Fire and Proteus have all but disappeared. (Adium even has an Import Fire Contacts menu choice.) Adium supports quite a few IM services:

* AOL Instant Messenger (Including ICQ and .Mac)
* Jabber (Including Google Talk and LiveJournal)
* MSN Messenger
* Yahoo! Messenger (Including Yahoo! Japan)
* Bonjour/Rendezvous (Compatible with iChat)
* Gadu-Gadu
* Novell Groupwise
* QQ
* Lotus Sametime.

Source: http://www.adiumx.com/