Adium is a free messaging application made specifically for Mac OSX. It's not only compatable with Microsoft Messenger accounts, but also AOL, Yahoo, ICQ, Google Talk, XMPP, Mobile Me, Bonjour, Myspace IM, Live Journal Talk, Facebook Chat, Lotus Sametime, Novell Groupwise, QQ, Gadu-gadu, and a few more that aren't listed on the Adium website. I'm sure you're probably not going to use all of those, or even know what half of them are. The great thing about Adium is that everything about it is customizable, from the contacts window, to the emoticon sets, to the message window styles, all downloadable for free on the official Adium webpage. Unfortunately, it does not have webcam capabilities.
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