You probably can't tell very well from the photo, but it is a metal sliding support for a computer keyboard that fastens to the underside of a flat surface.
And this for $4.99:
It's kind of a felty, plastic drawer divider which should hold all of my various and sundry stuff, once attached to the hanging support (I hope).
I made little marks on the underside of the "door" where I wanted the "drawer" to hang (you can see them if you look closely), then removed everything from my work table and flipped it upside-down. I got out my trusty electric drill and laid out the template included with the "drawer." Drilling into a hollow-core door is a bit tricky, and I had some trouble, but at last, the anchors were pounded in, and I was able to screw the whole thing into place!!!!.
I fastened the drawer liner onto the keyboard pad with double-stick carpet tape, and Yippee Skippy, it works perfectly!!!!! Can you tell I'm a bit proud of my handiwork? I'll have to be careful not to put anything too heavy in the drawer, or lean on it (as I am wont to do), but on the whole, this is exactly what I wanted--a place to store necessary stuff that took up too much room on my work surface.