MSN Messenger

MSN Messenger is an instant messaging service with its own proprietary protocol
The official client is produced by Microsoft, and is available from http://messenger.msn.com/ for Windows or the Mac
The servers are run by Microsoft. Interoperation with Yahoo IM users is promised, but is not yet available to all users
MSN messenger is not the same as Windows Messenger, which seems to be intended for corporate intranet deployment
To confuse us further, Microsoft appears to be rebranding MSN messenger as "Windows Live Messenger"
Unofficial clients include the multi-protocol Gaim and Kopete; and the cross-platform aMSN (Alvaro's Messenger), which is written in Tcl/Tk