The Wikipedia: BeagleBone Black is $45 Linux computer -- 1 GHz ARM Cortex-A8, 512 MB DRAM, Ethernet, etc.
Replicape and BeagleBone
The Replicape includes all the electronics necessary to turn the BeagleBone Black into a dual-extruder RepRap controller: 5 DRV8825 stepper drivers, etc.
The Replicape was apparently originally designed for the thing-printer (formerly HipsterBot), but it should work with most RepRap hardware. http://hipstercircuits.com/category/replicape/ via http://www.adafruit.com/blog/2013/05/23/replicape-revision-a2-updates-for-beaglebone-black-3dthursday/
Apparently several other people have built 3D printer capes or other CNC capes for the BeagleBone. "An unofficial list of various 3D Printer, CNC, and motion control related capes".