I found out that you can start and stop a movie in QuickTime Player by clicking on the movie picture. A single-click will stop a playing movie, while a double-click will play/continue.
Furthermore, what I find very useful is that the QuickTime plug-in in browser windows can be toggled in this same manner.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050209100314843