Apart from iChat, that comes bundled with OS-X but can only connect to a limited number of networks, most OS-X versions of instant messaging clients, like MSN Messenger, are fairly poor compared to their PC counterparts. An interesting alternative I discovered recently is Adium. It is quite basic in terms os features so far but it can connect to an impressive list of networks, including MSN, Yahoo, Google Chat, ICQ and quite a few obscure ones, which means that you only ever need one chat client open. It currently doesn't support voice or video but it's definitely on their to-do list. Nor does it support extended animated smileys like MSN does, although you could argue that's rather a good thing. A very nice feature is the ability to use tabs rather than individual windows for each thread of discussion. You can even have a hybrid setting whereby you have one window per group of contacts and tabs inside each window. And of course, in typical open source geeky fashion, most settings can be changed through the preferences and you can completely customise it using AppleScript.