Popup Window (US$5.99) provides an organizer utility for your desktop, enabling you to stick folders onto the edges of your screen and fold them in and out on demand. I loved the idea of this app although I ran into several problems when actually using it.
The app is meant for anyone who needs to balance easy folder access with the desire for a clean workspace. Drag a folder to the edge of your screen and it transforms into a tiny spring-loaded tab. Click the tab and the folder opens and closes using a clever accordion effect.
Unfortunately, material from the Popup Window does not necessarily act in the way that native folders do, as you can see in the following video where I attempted to drag to Imgur.
The developers wrote, "Popup Window imitates Finder file dragging, but of course Popup Window is not Finder. Unfortunately, we can't change how Safari works. Sorry." They pointed out that drag and drop does work with Dropbox, which I confirmed.
I also encountered problems with aliased folders (they did not work at all) and some folders on external drives. I'm not entirely sure what was going on there, but here's the screenshot. I suspect some of this is due to App Store limitations.
Popup Window shows early promise with a really intriguing underlying idea. I look forward to seeing the app mature over time.