So one of the awesome things about Snow Leopard is QuickTime X. One of the best things of QuickTime X is hardware accelerated playback. However, you can have it now if your computer supports Core Video. For Core Video support you need Tiger (10.4) or Leopard (10.5) and a programmable video card (basically most 64+ MB cards are programmable). If you don't have both of those, you won't get hardware accelerated video out of this, but you will end up with QuickTime able to play just about any video.
You need 3 things: NicePlayer, Perian, and Flip4Mac. NicePlayer will attempt to route the video though Core Video using the QuickTime libraries. Perian adds a whole slew of video formats to the QuickTime libraries, and Flip4Mac adds wmv support. After you install all three, QuickTime, and thus NicePlayer, will be able to play almost any video you can throw at it. If you use NicePlayer, the video will be processed by your video card instead of your CPU.
What does this mean? For older Macs especially, it means a significant improvement in playback performance. For example, my PowerBook G4 @ 1.25 GHz really struggles with 720p video and can't play it back smoothly unless I use NicePlayer to get it through the video card. If you have an almost brand-new Mac, I'd love to hear what kind of performance increase you get, if any. Also, installing all those codecs will allow your video editors which use QuickTime (iMovie, FCE, FCP) to handle more videos better.
NicePlayer also has the added benefit of being able to fill one screen without blacking out the other. I have 2 screens, and I often put a video up on my 2nd monitor while I chat on IM, browse the web, do finances, whatever. QuickTime, VLC, DVD Player (NicePlayer can do DVDs as well) all black out both screens when put in full screen mode.