The original idea was somewhat like enterprise blade server - a chassis that takes care of power and network distribution, with processor modules that can be easily added and removed. The recently released Raspberry Pi 2 features a quad core ARM processor, so they're very capable little machines for $35. Low energy usage (compared to a regular linux server) and fanlessness make them ideal!
You can download the 3d models here, created with SketchUp. To print, you'll need the STL Plugin to export a format that most 3d-printing host software likes. All the pieces (along with some prototype and test models) are in the one document.