Since yesterday, I'm using
Adium X as instant messenger client unter Mac OS X. Adium X is based on libgaim, and so I can finally throw away all the other IM clients I had running before as Adium X connects to Jabber, ICQ and MSN (and many more, but these are the protocols I use). It perfectly integrates into the OSX GUI, supports tabs, it looks really cool, and the duck logo is cute. And it calls itself "the best IM client ever". Well, I'm still missing some bits and pieces, but it's definitely better than the other clients I've used before, so I think I can agree on that.
Oh, one more thing: for a huge pile of nifty add-ons and extras, have a look at
www.adiumxtras.com.