Windows Live Messenger
(Formerly, MSN Messenger)
All of a sudden in 2001, I started noticing my friends were at their computers chatting away at that useless free messenger that came with Windows instead of the best of the best (at the time) - ICQ. I couldn't figure it out at the time, but I later learnt it originally came with free calling minutes, and it made sense that my friends on campus needed a way to call home. Especially, if home was overseas. The program later caught on as people's full-time Messenger client.
This free program was referred to as MSN Messenger, or simply, "MSN". Microsoft later renamed it to Windows Live Messenger. I notice, even with all these multi-messenger clients, Windows Live Messenger is the one and only up in Canada. That, and a Facebook account, and you are set up here. Many people don't even know what program AIM is.
Older versions of Windows come with Windows Messenger, but the new version you can download here is more user-friendly in notifying you about IMs and organizing your contact list. It's a Windows program, but there have been some Mac versions released.