Ok, let's list them.
Krikzz project.
There's another project whose creator wishes to remain anonymous for the time being, to avoid being flooded with questions about when it will be done. (No, I'm not involved in this project.)
A DIY project. Unclear what happened to it. http://chipmusic.org/forums/topic/13256 for-gbgbc/
Jose Torres' project. Vaporware. Nothing to see here, move on. Trust me. http://hackaday.com/2009/08/28/usb-gameboy-cart/
This project, which communicates directly with an SD card. Can't exactly be defined as an SD cartridge, since it requires a connection inside the Gameboy in it current rendition. http://zdoom.ic.cz/gameboy.html