I ran across a couple cool hidden prefs for the new Dock this afternoon:
enable-spring-load-actions-on-all-items
mouse-over-hilte-stack
The first of these causes all Dock items to spring-load. That is, if you hover over it while dragging, the item will open and be shown. So if you hover over an app, it comes to the front and you can drop things onto its window. If you hover over a folder, you can drop into it. If you hover over a file, it will open the file and bring that window to the front. You could, for instance, drag an HTML page onto a web browser, and then to a previous tab to load that page in that particular tab. Or, you could drag an image file to drop into a rich text document at a particular location all in one fell swoop. This is really cool!
$ defaults write com.apple.dock enable-spring-load-actions-on-all-items -boolean yes
$ killall Dock
The second pref causes the dock to draw a highlight hover effect for the grid view of a stack -- it's a sort-of white gradient thing that appears as you mouse over each icon in grid view, as seen in the image at right. It's kind of cool, but not really at all useful.
$ defaults write com.apple.dock mouse-over-hilte-stack -boolean yes
$ killall Dock
To reverse either command, repeat it with no at the end instead of yes, then kill the Dock again.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071027195419540