Here in Canada, everyone uses MSN Messenger (now called Windows Live Messenger). The best application I've found for MSN on OS X would have to be aMSN. Yes, it's ugly, even with skins designed to replicate the Windows MSN Messenger, but it comes closest to replicating the Windows version when it comes to features. It's the only one I've found so far that supports all of the more minor features, like offline messaging, and ink drawings. Not even the Microsoft-made MSN client for OS X can do this stuff yet, which is pretty disappointing since it seems to be the most aesthetically pleasing for a long-time Windows user, and has more of a Mac feel than aMSN.