Perian, the QuickTime plugin is being discontinued by its developers. Topher Kessler at CNET:
Unfortunately, the developers have decided to move on to other projects, and in a posting on the Perian Web site, said they will be halting support for the software after 90 days from its final release. The source code will then be posted to an open-source community (Google Code or GitHub), where others can try maintaining it if they so choose. There will be no official support for Perian in Apple’s upcoming OS X 10.8 Mountain Lion, but unless Apple changes the way the program is allowed to interface with QuickTime then it should continue to function, though there are no guarantees.
This plug-in used to be one of the first things I installed on a new Mac. While I’m sad to see it go, VLC is more than capable of handling things from now on.