QuickTime
QuickTime is both a media player and a plugin.
As a plugin, it's really only used for one major file - QuickTime Movie files, ending in .mov. Usually this means homemade movies by Macintosh users or most movie trailers.
I make sure that Windows Media Player is my default for most other audio and video files instead, and I certainly need to make sure QuickTime doesn't try to play Flash files online. It can't handle them.
As a Media Player, it's pretty nice and has that Apple feel. It seems like a simple player, without extra tools like a Library viewer or a "what's new" panel. This is commercial software for Windows users, but still works for free. Free users simply have to put up with a "Do you want to purchase?" screen when you open the program. This screen does not come up when using QuickTime as a plugin.
Please note that if you are looking just to be able to play QuickTime files because your current player is not compatible, I highly suggest the much smaller QuickTime Alternative program.