IMPORTANT: Microsoft is replacing Windows Live Messenger with Skype. All your contacts will be moved automatically. Take a look at The Complete Guide to Moving from Messenger to Skype for more information, alternatives and tips.
MSN Messenger was the original incarnation of the now named Messenger for Mac and it's now been superseded by it. However, if you still have Mac OSX 10.2.8 to 10.3.9 (although Microsoft recommend 10.3.3 or later) this is the only version of MSN for Mac that will work for you.
Firstly, like the latest version of Messenger for Mac, you should know that it doesn't work with webcams. This seems criminal I know, but Microsoft has seen fit only to allow those willing to upgrade to the corporate edition to use a webcam with it. However, the rest of it is pretty much as you'd expect. It is fitted with all the basic functions you'd want from Messenger and the application lets you organize your contacts into different groups, set your status, and use emoticons, etc. New conversations open up in a new window, which you can be moved around anywhere on the screen.
While it's fairly easy to check your Hotmail account thanks to the Mail button at the top of the interface, sending files and images to other users can be a bit slow. MSN Messenger is sufficient as a basic IM client, but we'd recommend upgrading your version of OS X and updating to the latest version of Messenger for Mac. Not only is it more secure, but it will also offer more features and a sleeker interface.
MSN Messenger is a decent if basic IM client for anyone with a Mac OS under 10.3.9 but the lack of webcam function is a major drawback