Shoutbox

With most browsers, the OS is sent in the UserAgent field or some such similar field in the HTTP request. If the OS isn't recognised, or that info isn't sent, the site will usually offer a manual choice.

Anyway, if that site detects you're on a Mac, it should go to the MSN Messenger for Mac page.

Edit
My UserAgent field is as follows:
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6 Web-Sniffer/1.0.21

This says I'm using a Mozilla-based browser (Firefox 1.0.6 specifically), on Windows NT 5.1 (Windows XP), and to favour US English pages (depite being British). And I think the Web-Sniffer thing is because I used http://web-sniffer.net/. A regular HTTP GET probably won't have that bit.