Shelf for a desktop computer

A shelf for a desktop computer is disclosed, as are shelf and desktop computer combinations. The desktop computer is generally of the type having a computing unit and a support arm that extends downward at an angle and tapers. The shelf has an open slot along a rear edge. The slot allows the shelf to be placed on the support arm, where it remains, cantilevered, extending generally horizontally from the support arm at a defined vertical position. The shelf is without external mounting hardware, support flanges, or other such elements, although in some embodiments, small, internal set screws may be included.