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:Launch Quartz Debug, the 'Window Info' window should appear. And it's way better than the old one.It has live preview,application grouping,Core Image profiling,a reveal window button,etc.
Mac OS X Hints