We all know that you can option-click the folder triangle in list view to expand the selected folder and all its sub-folders at once.
What I recently discovered is that if you select one or multiple folders and hit the right arrow key (at least on my Powerbook's keyboard), the one or multiple folders expand one level. Then hit the left arrow key and the one or multiple folders collapse.
You can also take this a step further and hold the option key with either of the arrows. You'll reproduce the expand/collapse a folder and its subfolders like the option-click trick, but you can extend this expansion/collapse to multiple selected folders.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030218164922494