Windows/Linux: VLC gets a lot of praise around here, but its Achilles' heel has long been a lack of hardware-accelerated playing, which means HD content choking up. A rough 1.1 preview offers GPU-accelerated video on Windows and Linux for curious testers.
If you're running a media centre, or just need VLC to notice and take heed of your awesome video card when it's trying to play 720p or 1080p content, you've likely got little to lose running a 1.1 preview release — if it crashes too often for you, well, that's probably just how it was with CPU-only video rendering. Contribute your crashes as bug reports, too, and you're helping a very worthy cause. Otherwise, be prepared to trade off a little occasional frustration for the benefit of smoother HD video. My own laptop running 1.1 pre 1 seemed to do just fine with a few 720p videos, with no crashes after scanning around three different files.
The 1.1 preview of VLC's GPU acceleration is available for Windows and Linux systems; while there's a Mac download at the link, it remains a CPU-only player.