I do a lot of editing with vi and came across a neat trick tonight. If you want to convert letters to upper or lower case without having to retype them, simply hold Fn + Shift + Page Up or Page Down. Whatever character is under the cursor will change case. This is useful to me since I sometimes have to change variable names to all upper or lower case. Holding the key combo down will do the complete line.
[robg adds: My desktop Mac lacks the Fn key, so I tried this hint with Shift + Page Up/Down alone, and it worked as described. Then I tested on my PowerBook, which does have the Fn key. Very strangely, I couldn't get this hint to work at all at first. A bit of digging revealed the answer: the PowerBook's terminal type was set to xterm-color (in Preferences, "Declare terminal type ($TERM) as"). Once I set it back to VT100, it worked as described. I think this trick is relying on VT100 terminal codes (someone will correct me if I'm wrong, I'm sure), so it failed when set to xterm-color. Also, it doesn't matter whether you use Page Up or Page Down -- either key merely toggles the case of the currently highlighted character.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040529050009800