Gaim is a popular multi-platform instant messaging client that supports many popular protocols. Gaim is open source and is released under the GNU General Public Licence. It runs on UNIX/Linux systems, Microsoft Windows, FreeBSD and Mac OSX as well. It is currently used on many distributions of Linux such as Knoppix, Ubuntu and others.
Here are some of the protocols that Gaim supports:
Google Talk (Through XMPP)
The best part about Gaim is its ability to allow users to log in to several IM accounts simultaneously. For instance, I could be chatting with someone using MSN and with a second person using ICQ and chat with a third person using Google Talk! Although there are some other multi-protocol clients such a Trillian that offer similiar functionality, I find that Gaim is the best in terms of ease of use and also due to the fact that its FREE!
In my opinion, Gaim is an extremely well-written piece of software. It is lightweight and ad-free. Gaim is also highly customizable through the use of downloadable plugins that add loads of functionality to this nifty piece of software. Only downside about Gaim is the file transfer function in MSN gives you really slow speed as Gaim cannot establish a direct connection between the 2 peers as of now. As of now, all connections are routed through Microsoft's servers.
I got to know about this software through using Linux systems and I've been using it on all of my machines ever since, even on my Windows boxes!
Download the Gaim instant messenger client complete with GTK+ from their project page at Sourceforge here:
Gaim Downloads (All Packages)
Gaim Downloads (Version 2.0.0 Beta 5)