Miranda IM is an open-source multi-protocol instant messenger client for windows. It's smaller, faster and easier!
For more details on Miranda click here

Download:-
Miranda v0.8.5 for Windows
size 1.63mb

Google talk is a chat client for the gmail server. Google talk is now Open-source.

Pidgin is an easy to use free chat client used by millions. You can connect to any multiple chat networks using this client i.e googletalk, yahoo, AIM could be connected simultaneously!