Here's one for the keyboard navigation lovers among us. After hitting Control-F3 to highlight the Dock, you can now go straight to the icon of your choice by typing its first letters (i.e. type "it" to jump straight to iTunes). You used to have to hit the arrow keys till you got there (ugh).
I love this -- I always thought it was missing in Tiger!
[robg adds: You'll need to have full keyboard access enabled -- press Control-F1 if Control-F3 doesn't seem to do anything. I honestly don't remember if 10.5 ships with this enabled or disabled by default.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071027171904185