AOL Instant Messenger: The official site of AOL Instant Messenger. Exchange messages with friends, family, and colleagues online

ICQ: Official site for the instant-messaging and chat software

Jabber Software Foundation: Not-for-profit organization that oversees the general development of XMPP and maintains the Jabber Enhancement Proposals

Yahoo Messenger: Allows you to instant message with anyone on the Yahoo server. Tells you when you get mail, and gives stock quotes