Pidgin always had the love of Linux users and due to its vast pool of plugins and multi network support (more than 16), its arguably one of the best chat client on windows as well.
It was originally created in 1998 with the name: Gaim. It’s completely free and open source. Did I tell you that its available for Windows, Mac OS X, Linux, BSD, Solaris and even Amiga!
The best part of Pidgin is that, unlike other desktop clients such as Digsby,Yahoo! etc. which show nagware and spam you to the core, Pidgin lets you do what you intend to: Chat Peacefully! It sits quietly in the tray so that it won’t come in your way.