Instant Bird is Multi Protocol Client supports of AIM aka AOL Instant Messenger, MSN Messenger aka Windows Live Messenger, YM aka Yahoo Messenger, GTalk aka Google Talk, Gadu gadu, ICQ, IRC, QQ, XMPP that works for Windows Platform, Mac OS X and also Linux.

Features available in InstantBird: Chat Room, Contact List, Tabbed IM Windows, and many more. It’s developed by Mozilla Teams that also create Mozilla FireFox, Thunderbird and more.
Download Page: InstantBird
License: Free / Open Source
Website: www.instantbird.com

GAIM, in the past is the name of the Pidgin Multi IM Client. Right now it turn into Pidgin. Just like Miranda IM, Pidgin is Multi Protocol Chat Client that support most of Messenger Services. That means you can login and use your account from Yahoo, Windows Live, Google, and other to chat using Pidgin.
Pidgin Supports AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr. You can also install a lot of plugin to make Pidgin …