There are quite a few accessories made to slap a Mac mini underneath your desk, but Macessity’s MiClassic Mount Bracket ($55) takes a different direction. Not only is it a tray to hold the headless Mac, but it also has a four-port USB hub built right in.
The biggest downside to this particular mounting solution is that the powered hub requires its own energy source. Macessity claims the power adapter is compact though, and the ability to charge a bunch of devices may be worth it. When combined with the $20 Swivel Bracket, it becomes an even better solution. Slide the bracket in place and you can spin your Mac mini around at will for quick access to the ports on the back.