Recently, I was inspired to make an inexpensive little GameBoy development cartridge for my own modest use. I was actually inspired by the creation of the new .GBS ripped music format: I wanted to play these on my real GB hardware.
Conveniently for me, I had a Gameboy cartridge that had an EPROM inside instead of the normal miniscule ROM (see here). Although Snow Bros. Jr. was a good game, I had played it enough, and the rewards from having a mini dev-system on the GB were greater than the loss of the game. I still have the EPROM from SBJr. intact, although the legs of the chip are very short, and I can't say if it still works.
Anyway, the fact that the SBJr. game had an EPROM inside meant that I could put in a full-size socketed chip, which really is too difficult to do for any other GB cart. Unfortunately, since I don't have EPROM equipment, just an EEPROM programmer, I soldered in a 28-pin socket, rewired some of the necessary address and power lines, and programmed a 32k EEPROM with Tetris as a test program. After removing the faceplate from my Super GameBoy (since a cart with a socketed EEPROM is thicker than the SGB will allow), the cartridge worked perfectly! I cleaned up the board a little bit by taping down the wires and exposed contacts, put some padding under the socket to keep it protected, and carved a hole in the front of my GB cartridge casing. Voilà! Easy and clean (sort of).
So, now I can use any 32k GB ROM in my devcart, including most demos and GBS music rips. You might say, "What a waste of a cartridge, since you can't play any larger games or GBC games! You should have just gotten a GB Exchanger or some other copier!" Well, I don't really care enough about the GB to buy a copier, and I don't really want to invest in piracy. 32k will suit me fine for now. I did this because I could (in fact, I could have done this last summer if I had thought about it). I think it's neat, and I'm surprised how easy it was to do.
To the right and above is a pic of my GB cartridge fully assembled inside the casing, and below is a picture of just the PCB.