The third-party video codec pack for QuickTime "Perian" has been updated to version 1.2.1, which adds support for more codecs and fix a number of bugs that caused crashes. The application is a popular plugin for QuickTime that allows many non-standard and obscure codecs to be played in Apple's default media player.
Specifically fixed are problems with the program crashing in OS X 10.4 when displaying subtitles, and playback problems with AVI files that were incompletely imported. In addition to these fixes, Perian now supports Indeo 5 decoding.
Perian is not the only option for viewing video formats that are not supported by default in QuickTime. There are also the free VLC player and MPlayer OSX Extended with the binary codec package (both updated in January) that will play a variety of files, and the official Windows Media codec for OS X, that is distributed in the "Flip4Mac" package.
These are very popular alternatives to QuickTime, and I recommend you give them a try if you are having problems viewing media with QuickTime. I have found the best practice is to keep two or three alternative media players available because each will handle formats and encodings differently, and the optimum one to use may change for a particular system and software setup.