I don’t use the Dock and have it hidden away in a dusty corner of my screen. This Tip though from Brad’s MacTips website is one I hadn’t known. [Update 2009: that site is no longer being updated.]
Open Application In Finder: To open and select an application in finder, Command Click an application in the dock. Now watch the finder window open in list view with the program selected.
Remember that you can do other things with the Dock too. Control Click an item to bring up a list of possible actions, such as Quitting an application. Option click to Hide and Show the current application. There’s a good list of keyboard commands at Apple’s website: Mac OS X 10.1: Additional Features of the Dock.
Update, October 2008. The Command click trick doesn’t seem to work in Snow Leopard, but there are other tricks available instead.