This may defeat the do-it-yourself object, here, a bit, but there's always the Retrode
. It's a USB reader for SNES and Mega Drive (Genesis for those in North America) cartridges, which also has SNES and Mega Drive joypad ports (two of each).
It presents the data from the game cartridge as a file, which can then be used with the appropriate emulator, and it presents connected joypads as standard USB Human Interface Devices.
It will also soon have add-on adapters for other types of cartridge (specifically, Game Boy/Game Boy Colour/Game Boy Advance, Master System, and Nintendo 64), and these are currently in the pre-ordering phase.
I'm not associated with the device - just a happy user.