I have used Gaim in Mac OS X via X11 and Darwinports :Pgnulinuxman said:Gaim for Linux. I like using only 1 program for everything.
Apple X11 :http://www.apple.com/macosx/features/x11/
Darwinports: http://darwinports.org/
screenshot of X11 on Mac OS X;
http://homepage.mac.com/sao1/PhotoAlbum9.html