On a Mac running Lion, in the Save As dialog boxes for seemingly all applications, there's a white popup box that appears and disappears on pressing the space bar. I notice it a lot as I use a graphics tablet with one of the pen buttons set to pan / scroll the page, which in adobe creative suite applications on a Mac, maps to the space bar everywhere in the application including OS dialogs - so I often trigger it by accident when trying to scroll through directories when saving files.
Here's an image of it:
It took a while to figure out it was the space bar that was bringing it up (since I wasn't ever actually pressing the space bar...). Its close 'x' button seems not to work, so until I figured out (just now, actually) that it was the space bar that toggled it, I couldn't get rid of it most times it unexpectedly popped up. Dragging files into it doesn't seem to do anything, which discounts my theory that it's some kind of collector for overwriting multiple files at once. Nor does clicking on files or folders to select them. What is it and what is it for?