In Mac OS X Leopard, there is a command-line option you can use to get a mouse-over effect when looking at Stacks. Here's an example of what the effect looks like (the mouse is over Chess.app and it's highlighted):

An example of an highlighted application in a Leopard Stack

To get this effect, you first need to open a Terminal Window (go to Applications > Utilities > Terminal). At the command line type the following:

defaults write com.apple.dock mouse-over-hilte-stack -boolean YES

and then press Enter. Then enter the following:

killall Dock

and press enter again. The effect will then be enabled. If you decide that you don't like the effect, you can repeat the above commands but in the first command, change the YES to a NO.