The best OS for Efika is - of course - MorphOS!
This is the efika board:

Compact: Width: 118mm Length: 153mm Height: 38mm
Efficient: Only a mere 4,1 Watt of power consumption (1080mW by CPU, 3 Watt by 128MB RAM)
A complete system with graphics card and hard disk consumes less than 20 Watt of power.
Thanks to Axel Knabe this project was initiated this fast. Many thanks to him for his hard work and time spending for this stuff. He also made the pictures on this site.
Frank Wille made, inspired by this case, his own case version for his Efika by using the same Icy-Box case.
Thanks to Jochen 'Roschmyr' Rhein a PDF containing the Efika back plate is now available. This PDF allows you to simply print out a mask for creating your own Efika case. Please note that the holes for graphic card may not match with your card, as there are several card designs out there, where the connectors are moved a few inches or have an entire different layout.