You can use the keyboard shortcut Command - Left Arrow to play QuickTime clips (sound and video) backwards. Command - Right Arrow plays forward (as does the space bar) ...
Maybe not the most useful hint, but kinda fun.
[Editor's note: There's an older hint that talks about a QuickTime throttle in the Finder, but this is different - and it works in both the Finder and QuickTime player. It might also pre-date OS X, but I didn't find it documented here, so here it is...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030205070948616