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MacPorts and Fink are basically like apt-get on OS X (it installs open-source software on Mac OS X)

Perian?

Perian, the Quicktime "all-in-one" codec pack, which allows Quicktime Player (And Quicktime based applications, including Front Row) to play various extra formats, including DivX/Xvid, .mkv etc etc.

The problem

One problem - well, more of an annoyance than a problem.. With version v1.0, it started being released as a Preference Pane icon - I never have to change it's settings, so it doesn't need to be in the preferences..