Created by Cerulean Studios, Trillian is an instant messaging application for Windows which can connect to AIM, ICQ, MSN Messenger, Yahoo, IRC and Jabber networks.
The program was originally available only as freeware, until on September 10, 2002 the first commercial version of Trillian, Trillian Pro 1.0, was released. The creators of Trillian have promised to continue to support and maintain a free version of Trillian. However, Trillian remains a closed-source application.
The program was named after a fictional character in The Hitchhiker's Guide to the Galaxy by Douglas Adams.
Cerulean Studios recently released version 2.0 of Trillian, and are working on revising the free version of Trillian.
Examples of other multi-protocol instant messaging clients for Windows include Miranda IM (open source) and jabber. For Linux and other Unix-like operating systems they include Kopete (KDE). The Milkbone and Gaim clients works on both Windows and Unix-like operating systems.
Like trillian both Gaim and Miranda IM support feature extensions via plugins. Miranda IM has a far more extensive-, varied-, and feature laden collection of plugins than trillian. Miranda IM plugins, like Miranda IM itself are free.
See also: designHazard, who designed Trillian's default skinAlternate Multiprotocol Clients