The great thing about having an open market is that apps that might not be accepted in other app stores have a clear path for release in Android Market. For example, there are emulators galore for the iPhone yet you won't find them in the App Store, you'll have to jailbreak it to take advantage. With the T-Mobile G1 and Android? Well you can download a Game Boy Emulator right in Android Market. Hint: It's called AndroidBoy.
AndroidBoy is a fairly basic emulator that supports Game Boy and Game Boy Color games. There doesn't look to be a d-pad so the controls may be a little different than what you're used to. Either way, emulators are fun so go have fun and download AndroidBoy!