Open Source for Mac OS X

Adium is a free and open source instant messaging application for  Mac OS X.  It can connect you to any number of messaging accounts on any combination of supported messaging services (MSN Messenger, AOL Instant Messenger and Google Talk to name a few) and then chat with other people using those services.

Adium is a third-party client for connecting to services run by other people (mostly companies). And thus does not host any IM services themselves.