Here's an interesting Dock modification, though to be honest, I'm not sure exactly when it might prove useful. In Terminal, enter these two commands:
$ defaults write com.apple.dock static-only -bool TRUE
$ killall Dock
As soon as you kill the Dock, when it respawns you'll notice that only your running applications are listed -- in other words, any program you've added to the Dock won't be shown unless it's running. You can obviously accomplish the same end result by simply dragging all your static applications out of the Dock while nothing other than the Finder is running; so that's why I'm not sure how useful this command really is.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090108060519304