[19-Dec-07: The offer has apparently now expired, sorry! -Adam]
For an unspecified limited time, DivX is giving away free serial numbers to their DivX Pro software, which makes it possible to create DivX-encoded videos on the Mac (a Windows version is also available). DivX is a compressed digital media format for video, much as MP3 and AAC are compressed digital media formats for audio. Read all about it on Wikipedia if you're interested in the gory details.
Normally $19.99, DivX Pro for Mac is a bundle of four applications, DivX Player (for playing DivX videos on the Mac), DivX Web Player (for extending playback functionality to the Web), DivX Pro Codec (necessary for encoding video in DivX format), and DivX Converter (the application that works with the codec to create DivX videos).
If you don't create video at all, there's nothing wrong with downloading the free version of DivX Pro, but you can get just the parts you need to play DivX-encoded videos for free any time. The DivX for Mac bundle includes the DivX Player, DivX Web Player, and the DivX Community Codec, which I gather can only decode DivX video, with encoding being restricted to the DivX Pro Codec.