Some fans of the Raspberry Pi just can’t get enough, it seems, and have devised various ways to mount multiple Pis. In fact, when this idea occurred to us, we thought we had the perfect name for such a device – the Pi Rack – until we found out that name was taken by hydroponic growing systems that are sometimes used to grow medical marijuana, and other types of plants. So, guess that idea was only half-baked.

(We’ll pause a moment while you groan at that bad pun, realizing it works two ways. You know, baked pi(e), and baked from… do we really need to spell it out?)

Anyway, we came across this clever way to mount multiple Raspberry Pis, using a block of plywood and the audio and video jacks as mounts:

For more photos, and details on construction and the components used, see this article and the comments underneath:
Simple Pi Rack (raspberrypi.org)

(2020 Edit:) If you happen to stumble across this old article, note there have been several newer ways to mount multiple Pi’s developed since it was written. Some are homemade and some commercial. Some require a 3D printer, such as this one:

3D Printed Raspberry Pi Server Serves Up Fourteen Slices of Pi (Tom’s Hardware)