There are a lot of options out there for the instant messaging crowd. There are probably three multi-protocol clients available for every one single-protocol (standalone) client. There are two related multi-protocol clients, that work pretty much across the board on the three major flavors of operating systems, that I think are flat out better than anything else out there.
If I’m installing an IM client on a Windows or Linux box I’m going with Pidgin. Pidgin is released under GPLv2, and has implemented (at present count) 16 IM protocols, ranging from the standard ones — MSN, Yahoo!, AIM, and Jabber — to the no-so-standard — Gadu-Gadu, MySpaceIM, and Groupwise to name a few. While this does require the GTK libraries to be installed on Windows, they have seamlessly integrated this into the installer, so there is no pain involved. There are lots of plugins available, including my favorite — Psychic Mode.
Adium is a cousin of Pidgin. Adium is built on the libraries behind Pidgin, and is only for Mac OS X. Where Pidgin doesn’t quite seem to carry the look and feel of Windows, Adium is 100% Mac in its look and feel. Adium also integrates perfectly with Growl. The protocol support is a little bit less with Adium compared to Pidgin, but I’ll never use the obscure ones it doesn’t support. It does, however, pickup support for the .Mac protocol.
You may say, “What about Trillian? What about iChat?” What about them? With Trillian, you have to buy the pro version to get the Jabber protocol. With iChat you only have .Mac, Jabber, AIM, and Bonjour protocols; no MSN or Yahoo! support. I’ve tried using Trillian and iChat, but I always end up back with Pidgin and Adium.
iChat does have one major advantage over Adium at this point: voice and video chat. While Adium (and Pidgin) is currently working on this issue, you have to go to iChat to take advantage of that functionality.