Grbl fits in a ATmega328

https://github.com/grbl/grbl/downloads

https://github.com/grbl/grbl/wiki/Flashing-Grbl-to-an-Arduino

http://dank.bengler.no/-/page/show/5471_gettinggrbl

http://dl.dropboxusercontent.com/u/54312401/grbl/grblflash

GRBL Arduino Library – Use the Arduino IDE to flash GRBL directly to your Arduino: http://blog.protoneer.co.nz/grbl-arduino-library/

https://github.com/simen/grbl

ShapeOko Grbl settings calculator

Z Axis Mod for Shapeoko

http://opensourceecology.org/wiki/GRBL_Config   How to Modify GRBL Settings

https://github.com/damellis/gctrl   gctrl: a simple GUI for grbl

https://www.synthetos.com/webstore/index.php/assembled-electronics/grblshield-grbl-arduino-diy-cnc-shield.html





Here you can find a GRBL fork for Printrboard and Teensylu:
http://blog.lincomatic.com/?p=564
https://github.com/lincomatic/grbl