Instant Messaging (IM) tools allow a user to send a message that will "pop up" on the screen of another user on the network.
Instant Messaging tools with clients that run on the Macintosh include:
Predecessors of IM are "talk", Internet Relay Chat (IRC)...
MSN has stopped to support Mac OS 9.1 and erlier, the only systems which run on beige Macintoshes. Then if you have a beige Macintosh and want to connect directely to MSN, (probably) your only option is Mac Messenger.