The Elevation Shelf is a small, plastic (but very strong) shelf that you can stick to the underside of your desk in seconds with four built-in 3M adhesive strips. If the bottom of your desk is unfinished, or if you need the shelf to hold more than a couple pounds, it also includes screws and a small drill bit to really affix it in place.
Cleverly, Elevation Lab included a tiny lip on the front to keep pens and pencils from rolling out, and a small hole in the back lets you run cords to charge your mouse, your phone, or even a power a Qi pad. An adhesive cable clip also tags along in the box to keep said cords out of sight, if they have a lot of extra slack.
In hindsight, the Elevation Shelf is about as simple and obvious as a product can be. And yet, there are surprisingly few low profile under-desk storage options out there, and those that do exist require drilling into your desk, which may be frowned upon your office. In fact, I’d say 99% of desk organization products are things that go on top of your desk, but for small items like your wallet and keys, utilizing the oft-wasted area under your desk is a much better use of space.