If you are looking for an open source Instant messenger client for Windows or Linux then you can try Pidgin. Pidgin is a free instant messaging application which will help you to talk with your friends on AIM, Jabber/XMPP, MSN Messenger, ICQ, Yahoo!, QQ, Lotus Sametime, Bonjour, gadu-gadu, SILC, SIMPLE, IRC, MySpaceIM, Zephyr and Novell GroupWise Messenger. The good thing I liked is that you can login to multiple accounts simultaneously. So you can chat with your friends on yahoo and at the same time chat with partners on Jabber.
Pidgin supports many basic features like file transfers, away messages, different networks and typing notification. Apart from these Pidgin also supports Buddy Pounces, which helps you to configure different sounds for user status. Pidgin also supports buddy ticker, spell checking, tabbed conversations, text replacement and many others via plugins.
Pidgin runs on Windows and Linux. It can integrate with system tray on Windows and also GNOME 2 and KDE 3.1.