- 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