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Adium is a free and open source multi-protocol instant messaging client for your Mac. Adium allows you to integrate all your instant messaging accounts into one. Adium supports: MSN, Yahoo, Facebook, Jabber, Twitter, ICQ & much much more.
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Boxee is a free Home Theatre PC application. Boxee has a nice looking and easy to use UI, supports various audio & video file formats, integration to on-demand internet video streaming, 1080p HD support and much more.
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The Unarchiver is a simple, free and open source data decompression utility that is more featured than the standard Archive Utility in Mac OS X. The Unarchiver supports multiple file formats such as: Zip, Tar-GZip, Tar-BZip2, RAR, 7-zip, LhA, StuffIt and much more.