There’s a bounty of beta Game Boy cartridges on eBay. Seller devwan has prototype cartridges for Heroes of Might & Magic 2, Army Men 2 and Games Frenzy, which was only released in Europe online. The games aren’t that interesting unless you’ve been fishing for a copy of Rox. However, he has two items that might be of interest to collectors: a lot of three blank rewritable Game Boy Advance development cartridges currently and the Midiboy. The Midiboy plugs into a Game Boy Color and it is used to develop midis for it. While it might be neat to play with if you’re into homebrew, it looks like it needs to connect with a PC to get full use out of it.