Mac OS X only: Freeware program NicePlayer is a media player with an innovative interface designed for "people who like to watch movies."
The highlights: The playback window's size is adjustable with your scrollwheel, the control overlay only appears when you hover over the screen (kind of like iTunes' video control), and of course, NicePlayer has a fullscreen mode. The player still uses the Quicktime engine to power your video, so you might still want to download Perian to get all your video codecs working.
Oh, and NicePlayer has built-in support for the Apple Remote (Menu button toggles fullscreen, which I love), which kind of beats out both VLC and Quicktime (short of Front Row). I like it.
NicePlayer [niceplayer.indyjt.com]