While I don't really use I'm very much, I do like Trillian for Windows. When I switched to OS X for my personal use, I was disappointed to find out that Trillian wasn't available.
I also found that I don't particularly like Yahoo messenger on OS X.
So, I was thrilled to learn of this little open source program. Fire.
https://fire.sourceforge.net/
I have been using it for a couple of weeks now. It seems to work perfectly well, from my point of view. I have MSN, Yahoo, and AIM open all the time. However, AIM doesn't automatically log on for some reason. Since I am not particularly interested in AIM, that really isn't a problem.
I strongly recommend that any OS X user try this program.
Fire was released under the GNU GPL. Its' backbone is built around GPL"d libraries, including firetalk, libicq2000, libmsn, jabber, and libyahoo2 (Linux libraries).