Suckers never die, they just exchange places with each other - A proberb
The short servers are all in the Tank GT-14 line, the one above is the Tank GT14-B5372-LC, basically a dual Xeon Tempest i5000VS board in a half length 1U case.

Should you want, you can take two of these and slide them into a rack from opposite directions allowing for 4S/U densities.
Moving to the AMD side we have the Tyan 4980, a 4S EATX form factor Opteron board. This is an incredibly compact 4S board, and to get it in that space, you lose a little, but if you want small, this is it.
The board is based on an Nforce 3600 Pro chipset, has 16 DIMM slots and six SATA ports. Where the size crunch starts to be noticed is the single PCIe slot in the top left. Other than that, you have everything you really need for a server, and it fits in a standard desktop case. µ