It's many times faster than QuickTime Pro, works with almost all popular video formats, and it's infinitely free-er.
It's also really easy.
Just drag in your file, and click Start.
Transmission is open source (MIT license) and runs on Mac OS X (Cocoa interface), Linux/NetBSD/FreeBSD/OpenBSD (GTK+ interface) and BeOS (native interface).
VLC - a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols.