Adium

Adium screenshots

Adium is a popular and, probably, the best instant messaging client for you Mac.

Alternative downloads

JBuddy Messenger
3.3
Java-based chat client. If you need to communicate with contacts on several public IM channels simultaneously and don't want to clutter your desktop.....
Softros LAN messenger
3.3
FREE
LAN messaging for safe, secure and effective intra-office communication.
NAVER talk lite
0.9
FREE
Naver Talk is an easy to use chat application for Mac.