If you double-click a print job in a printer's queue, it will open a Quick Look window showing the printing preview of that job. Especially useful when the file names don't have descriptive titles.
[crarko adds: I tested this, and it works as described. Simple, fun, and I would not have guessed to try this.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110523124437800