Adium (Mac, Free)
The best multi-protocol IM client for Mac I've come across. -Sanders
Pidgin (Windows/Linux, open source)
Formerly known as GAIM (until America Online got uppity and told them to change it). A small, free multi-protocol IM client. Keeps all your GAIM settings (on windows anyway) if you install it while GAIM is installed, which I thought was nice. Universal IM client like Trillian. -Vagabond Aeon
mIRC (Windows, Free/Pay)
Everyone's favourite for IRC. Easy to use and has some advance features. With practice you will get used to using it easily and efficiently. - CrabNebula
XChat (Multiplatform, Free)
Sweet and simple. If you dislike mIRC for some reason, use this. XChat has a simple interface and fairly customizable. - CrabNebula
Trillian (Windows, Free/Pay) (Cost: $30)
Universal IM client, supports AIM, YIM, MSN, IRC, ICQ. Pro version also contains support for Jabber. - Kikuichimonji
The best multi-protocol IM client for Mac I've come across. -Sanders
Pidgin (Windows/Linux, open source)
Formerly known as GAIM (until America Online got uppity and told them to change it). A small, free multi-protocol IM client. Keeps all your GAIM settings (on windows anyway) if you install it while GAIM is installed, which I thought was nice. Universal IM client like Trillian. -Vagabond Aeon
mIRC (Windows, Free/Pay)
Everyone's favourite for IRC. Easy to use and has some advance features. With practice you will get used to using it easily and efficiently. - CrabNebula
XChat (Multiplatform, Free)
Sweet and simple. If you dislike mIRC for some reason, use this. XChat has a simple interface and fairly customizable. - CrabNebula
Trillian (Windows, Free/Pay) (Cost: $30)
Universal IM client, supports AIM, YIM, MSN, IRC, ICQ. Pro version also contains support for Jabber. - Kikuichimonji