This is cool and all, but what's the utility? You can already isolate individual windows and arbitrary screen regions with the built-in OS X screenshot grabber.
You have a webpage open in firefox, safari, IE (via parallels). You grab all three in one go and reposition them on top of each other to get pixel accurate offsets?
Agreed. While I can see that it's a clever thing and nicely done, I can't think of anyone who routinely needs to screenshot all (or at least multiple) windows at once but separately.
Someone who saves a ton of time using this, please tell us what you're using it for!
As someone who does a lot of UI mockups, something like this would be extremely handy. As someone else pointed out, if this saved me an hour of my life, it would pay for itself. If it saved me two hours, it would be saving me money. And an hour of set-up time occupied by taking and assembling selected separate shots of various tool palettes, settings windows, and main editor screens, for instance, would not be out of the question.
It's the "one fell swoop" aspect that is appealing. Just being able to open every possible window in an app and then hit "capture this to Photoshop" when starting on a UI clean-up pass mockup, or for creating a master document of all windows when taking a desktop app from 1st pass programmer art to shipping resources, and sorting them into Photoshop layers and folders, would skip a lot of currently requisite document setup/prep time.
The suggestion to use this to more quickly build manuals and tutorials is also spot on. Grabbing the current state of all windows in an app at "step 3" of a tutorial, and then being able to nudge them around to best facilitate the required tutorial/manual text, without having to manually capture all elements separately, is pretty sweet.
I just sent it to someone who publishes Photoshop tutorials for a magazine--lots of shots involve multiple windows and the layout people love to be able to play around with that sort of stuff.