I'm fairly sure this is new to Tiger...
You can now Control-click (or Right-click) on items in the Sidebar in the Finder and perform a number of actions on them. For example, you can click on a volume and eject it, or 'Open Enclosing Folder,' etc. Very handy.
[robg adds: This was one of my bigger complaints about the 10.3 sidebar; items put there were essentially useless, other than for clicking. Now you can do most anything to a Sidebar item via its contextual menu. The Desktop item even has an entry for Change Desktop Background, which takes you to the Desktop & Screen Saver System Preferences panel.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050503185517159