Window Inspector is a free program which lists all your desktop windows, and most of their properties: class, text, size of its various elements, owning process, command line, and a whole lot more.
It's very easy to use. Just move your mouse cursor over a window and the display updates immediately with the new values.
You can also "lock" the program to a particular window, then watch the values update in real time.
Window Inspector is mostly for developers, but it can also be occasionally handy for everyone else.
Have you ever seen a pop-up alert and not known which program launched it, for instance? Window Inspector may be able to show its owner.
The program also displays a tree of every window on your desktop, along with its various properties, and again this can be very informative.
Would you know what the window "avgnt.exe" was, for instance? Expanding that window showed us the text "Avira Free Antivirus - SysTray", so it looks like the Avira system tray icon.
A "Protection Cloud" branch of the same tree also displays individual prompts like "uploading", "filename" and "please wait while the program started is being analyzed". Even if you've never heard of Avira or Protection Cloud, you can see from this that the process is probably going to look for programs being launched, upload and analyze them somewhere.
Window Inspector displays a vast amount of low-level detail on desktop windows. Most of this is strictly developers-only, but if you can find your way around Process Hacker or Process Explorer then you'll find useful information here, too.