A friend of mine set out to build a box for running his test environment.

The goal was to build a small and quiet box, that didnt use much power either. He found a complete system, which is based on mini-ITX standards, with the brand new Intel Atom 330 Dual Core. (See bottom of this post for links to the hardware described)

ESXi Free was chosen as the hypervisor. (the box gotta have som style right?)
That however proved a small challenge in regards to the onboard NIC (realtek...) which is not supported by ESXi. Since this is a very small box, only 1 PCI expansion is allowed which required a PCI-riser card.
He went with an old Intel 100Mbit adapter. There is no reason why a 1Gbit card should't work, this was just a cheaper solution.

Since the casing is very small, it only supports 1x3.5" HDD. A mod was done to accomodate 2x3.5" HDD. That mod means that there is no longer space for a slim-line CD/DVD-Drive. So if you need that old and slow kind of storage devices, this mod is a no-go :)
He made his ESXi boot of external USB storage, which was done by following this guide. This makes it far easier to change HDD (when/if u have 2...) without having to re-install the hypervisor.

Performance on the box is pretty decent but dont expect it to redefine speed though. 3-4 VMs with 512MB memory each should not be a problem, especially not when you take advantage of memory page-sharing.

Power consumption is down to only 35w, and thats with 1 x 500GB SATA drive.

The price for running this polar-bear-hugging-co2-friendly mini server a year:

( 35 / 1000 ) * hours_running * kw_price (Which in my friends case is approx. 600 dkr a year)

Hardware: (approx prices in dkr)
PROconsult Mini ITX - Intel Atom 330 (2600 dkr)
Official PCI riser card (MiniPC.DE) (150 dkr)

Some pictures: