There have been three devcart efforts so far.  You can find the older information at:

http://work.playpower.org/w/page/17230536/SUBOR-Cartridges

 

DevCart 1:

 

Noah Vawter's Atemga32-on-breadboard Based  Cartridge Reader

 

 

DevCart 2:

 

tech t.'s atmega128 dev kit cartridge reader

 

 

 

DevCart 3:

 

NES Style?