Here's a quickie ... while hovering your mouse over the horizontal scrollbar in Panther's Save and Open dialogs, you can use the mouse wheel to scroll left and right.
[robg adds: I don't recall that this worked in Jag, and there are no hints about it, so I think it's new...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031012013331284