Miranda IM is an open-source instant messenger.
It’s a multi protocol messenger meaning you can login to AIM, Yahoo, MSN, GTalk, Jabber, IRC, ICQ, Gadu-Gadu and many more networks from this single software. It has a very robust and dedicated development team with a wide user base and a very active forum. I have used other multi-protocol IMs in the past like GAIM (now pidgin) but this one is the best in terms of expandability and customizability.
It has a HUGE range of plugins including skins, themes and functional plugins for upcoming protocols, chat history management etc. In addition to chat, it also supports file transfer and web page sharing. The Miranda IM setup file is pretty small, around 1.4MB, and can easily fit on a floppy disk. Miranda stores all its profiles, settings, history in a single encrypted database file which makes it portable and can be easily copied into and used from a USB thumb drive and the like.
Miranda also has extensive support for skinning and themes which is a good thing for people who like to make their IMs unique looking. Their addons page is a good place to start for plugins and customizations like sounds, skins, themes etc. All in all, it’s the best free multi-protocol messenger I have used.