Many manufacturers made pizza-box workstations, most notably Hewlett Packard, NeXT and Sun. These make a worthwhile addition to any esoteric hardware collection, and never fail to get a funny look out of gamer kids who (too often) seem to think that computers are by necessity either beige towers or bling-blingified LED-festooned shiny monstrosities that have more chrome than actual functionality.
Although they are somewhat old, pizza-box machines tend to run cool, are fairly quiet and have a rather small desktop footprint, while still remaining good for typical tasks such as web browsing and e-mail.
Most any of their native OSes can be configured to provide a decent user experience, though it might be worth trying Linux or BSD as well.