Quartz Debug is a part of the Apple Developer Tools and may be found in /Developer/Applications/Performance Tools. It's used to test and optimize graphics performance when writing a new Cocoa program. One thing bugging me with Quartz Debug version 4.0 in Snow Leopard is that the window list which listed all the open windows on the system along with their id, size, and memory consumption statistics is apparently gone. This was quite handy when debugging an application that uses multiple windows.
I say apparently gone because it's still there and this can easily verified by taking a look in Quartz Debug's Resources folder.There are lots of .nib in there not present in the old version. Here is how to restore the older behavior.
Open a Terminal prompt and type:
defaults write com.apple.QuartzDebug QDVisibleClientIdentifiers -array-add -string QDWindowInfoClient
You can also try this:
defaults write com.apple.QuartzDebug QDEnableAppleInternal -bool true
You can activate a mode which show you window information on mouse over+Cmd+Option with this:
defaults write com.apple.QuartzDebug QDShowWindowInfoOnMouseOver -bool true
[crarko adds: I haven't tested this one.]

