Skype Make calls anywhere in the world over the Internet.

Google Talk Talk and IM with your friends for free.

Gaim Keep in touch with all your friends on this GTK2-based instant messenger application.

Miranda

Trillian Chat with all your friends on mIRC, AIM, ICQ, MSN, and Yahoo Messenger.

meebo.com web based trillian so you dont have to download anything to use it and talk to your freinds from anywhere