AIM, Bonjour, Gadu-Gadu, Google Talk, Groupwise, ICQ, IRC, MSN, MySpaceIM, QQ, SILC, SIMPLE, Sametime, XMPP, Yahoo! & Zephyr!
Do notice it says IRC in the list! It's not as good as a regular IRC client, but it looks solid and it works. To the left example screenshots of an IRC channel and buddy list. (Black window borders are from Windows Theming.)
Pidgin is FREE and available on Windows and UNIX platforms!
There are a dozen plugins available for various functions. (Default Pidgin is rather plain.) Here you can get a plugin pack with 20 odd plugins, a must-get really. Go here if you want to add toaster pop-ups to Pidgin.
Musictracker displays what music you are listening to, it supports the following windows players: Winamp, Windows Media Player, iTunes and Foobar2000. On UNIX it knows how to communicate with Amarok, Rhythmbox, Audacious, XMMS, MPC/MPD, Exaile, Banshee, Quod Libet.
A small list of third-party plugins can be found on the Pidgin homepage. SourceForge still has a lot of older plug-ins, these haven't been updated in a long time. Use at your own risk.
Over here you can find a nifty Meebo widget plugin for Pidgin. It handles the authorization and deletion of Meebo guest accounts inside Pidgin. With this plugin you can use the Meebo Me widget generator located here.