An Open Instant Messaging Platform called Jabber

Jabber is an open instant messaging technology that is used by several free IM services. There are several free clients available on this platform and several servers as well, if you intend to run one in your own organization.
Jabber was acquired by Cisco in September 2008.
XMPP
Jabber created the popular Extensible Messaging and Presence Protocol (XMPP) [...]


Trillian: All in One Instant Messenger

Trillian is a fully featured instant messenger that supports ICQ, MSN, Yahoo Messenger, and IRC. It supports audio chat, file transfers, group chats, chat rooms, direct connection (AIM), proxy support, encrypted messaging (AIM/ICQ), SMS support, and privacy settings besides several other features.
If you have been running different IM software till now, switch to Trillian and [...]


Digsby = IM + Social Networking + Email

Digsby is a multi-protocol instant messaging which also supports e-mail notifications and alerts from MySpace, Facebook, Twitter and LinkedIn.
It uses Webkit for rendering, and has a slick interface. It requires registration and this can be done via the Installer or on the Digsby website. The User Interface is quite slick and appealing.

Digsby is available on [...]