This is not really a hint, but I think it's worth mentioning. The Finder preferences in 10.5 (Finder » Preferences) allow you to turn off the annoying dialog box that normally pops up when you change the extension of a filename. You'll find the setting on the Advanced tab.
[robg adds: The other new addition there is a checkbox to always securely empty the trash.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071205085528996