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.
Cape-RAMPS For BeagleBone: CRAMPS
Other 3D printer capes for BeagleBone
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".
Other 32-bit 3D printer controllers
Vaporware Electronics lists many other 32-bit processors and boards designed around them that people are considering for 3D printer control.