Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
Lingo - Empower Creation and Ensure Brand Consistency. Build and Share libraries of visual assets with your entire team — all from your desktop.
Gajim - Full featured and easy to use Jabber client
tSIP - tomeko.net/software: SIP client for Windows.