Window Seizer is a developer-oriented tool which displays low-level information about all the windows open on your desktop.
By default this data includes the window caption, memory use, handle, class, parent, process ID, file name, and the window's size and position.
The initial table only displays data on the windows you can see. Check the "Show Hidden" box, though, and after a sometimes lengthy delay you'll also see the many more hidden windows also active on your system. Which can be very useful for developers, but has more general applications for expert Windows users, too: if a hidden window keeps locking up, say, or grabbing all your RAM, you can spot it here and more quickly figure out what's going on.
And as well as watching your system, Window Seizer can also manipulate windows in various ways. So you can close, hide or reveal them, terminate locked programs, move them top-left of the screen (very handy if they're currently not visible at all), and more.
There are a few problems with the program. Memory use doesn't always appear to be reported accurately, for instance. And occasional lengthy pauses sometimes leave you wondering if the program has locked up. Still, Windows Seizer does have plenty of useful features, and as it's also small, portable and entirely free, you may find it a handy way to monitor exactly what's currently running on your desktop.