Update (2x): VideoLAN has issued some clarification on this development (read below)
VideoLAN Client, or VLC, has been a popular media player for OS X that has developed and progressed over the years. It has been a great alternative to QuickTime because of its support for a vast array of codecs and robust capability to play even corrupt videos. If a file has been damaged or improperly encoded, when QuickTime displays errors and does not play the file, VLC may be able to work. Unfortunately, at this point because of the lack of Mac developers helping the project, support for OS X is nearly dead.
No community support
Like all open source projects, VLC relies on community contribution for testing, developing, and financing. The project is lucky to have the backing of the VideoLAN non-profit organization, but it still relies on the community for developers, testers, and financial contributions.
Since May of this year, VLC has been desperately looking for Mac developers to help out on the project, but it appears none have stepped up to lend a hand. As of this month there are basically no Mac developers working on VLC, and as a result the project is nearing the end of development for OS X. This is bizarre to have happen given the rising popularity of the Mac; however, this is the nature of open-source projects, which require the active support of the community.
I feel it would be a shame to see VLC disappear for OS X, as it has always been a popular option that works when other players cannot play a specific file. Unlike many players that are just wrappers for Apple's QuickTime technology, VLC does not rely on QuickTime for playing videos and instead implements its own method of managing codecs and media decoding.
There are a few other alternatives to QuickTime, but the only one that compares to VLC is "MPlayer OSX Extended", which is the OS X version of the popular mplayer for Linux. It has been in development for a while and offers a number of features that are comparable to VLC. You can check out MPlayer OSX here: http://mplayerosx.sttz.ch/
Beyond MPlayer, there are a number of codecs and codec packs that can be installed to provide expanded file format support to QuickTime player. A number of these can be found on the Apple QuickTime Components webpage, but others that support Quicktime are also available.
Despite these options, VLC has been a great all-in-one player for viewing media in OS X. You can read more about this development (or lack thereof) at the VLC forums (1, 2), and read more about the VLC project at their website. If you have not tried VLC, and have had troubles with playing various movie and audio files, I suggest you give VLC a shot: http://www.videolan.org
UPDATE (12/17/2009 11:03AM):
The VLC forums have been overrun with requests about this, and they have issued a statement clarifying that the lack of development pertains to the Mac interface for VLC, and not to the overall VLC codebase. They are still actively developing it, but are having issues with some Mac details such as 64-bit support and the GUI.
In their initial forum postings they mentioned how the lack of Mac developers (current number being effectively "zero") would make version 1.1 of VLC the last version for the Mac, and they have been in need of Mac developers for some of the technological advancements they are implementing in upcoming versions. This suggests Mac development has dwindled; however, if the underlying codebase is still being actively developed, hopefully we will continue to see VLC on the Mac for years to come. Despite this, at this point the situation seems to depend primarily on whether or not they can get a good Mac interface developer.
UPDATE (12/19/2009 11:03AM):
When these articles about VLC came out, the VideoLAN group issued a statement mentioning the news about the Mac player's demise was greatly exaggerated, and issued statements about the promising "Lunettes" interface for VLC. Today their forums are back online and while they have provided links to the Lunettes source code, they have again reiterated that VLC for Mac may be dead on the Mac in the future, further substantiating the initial reports of the problem. A few developers have contacted VLC and offered to help, so while future VLC support for Mac is uncertain, there may be some effort brewing to keep it going.