Pidgin - most popular, free, multi-platform IM client that supports most of the IM protocols ,preloaded on linux , easily avilable for downloading for windows and mac
Miranda IM - lightweight, resource efficient and easy to use client with powerful plugin support
Universal Messenger Plus - a single convenient interface that allows you to easily communicate over different protocols.
Qnext - share your music, photos, and files with anyone, instantly while talking via IM
Adium - free client for Mac users that can connect to most of the popular messaging services
BitlBee - IRC clients with support of IM protocols
Ayttm - in case of a service failure, this will automatically fall back to other service protocols of the same person you are chatting to.
Trillian - a Windows application that can connect to multiple IM services, with nice features like Metacontact, Instant lookup, and Emotiblips
meetro - find and chat with people nearby, i.e. location-aware client
IMVU - 3D chat which enables you chat in 3D scenes with your own avatars
Kopete - KDE application, provides users with a single, easy-to-use way to access all of their instant messaging systems
MECA - connect to all of your friends on AIM, MSN, Yahoo! and ICQ from just one list
Centericq - text based multi protocols IM client
Proteus - popular client for Mac OS X that supports multiple protocols
Instan-t - Windows client that allows users to communicate with MSN, AIM, Y!Messenger, ICQ and its IM networks
iChat - IM Client from Apple for Mac OSX