If in Apple Mail, you select an attachment in a message and hit the Space Bar, Quick Look opens a window to let you view the file. Very convenient. AFAICS, this is not documented anywhere.
[robg adds: Note that this is different than using the Quick Look button in the Attachments section of the header, which is obviously documented via that big button. This hint refers to the actual attachment within the body of the message.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071031040055280