Here's an idea.... you know those NES and SNES test carts? The ones with the test screen for the controller inputs?
There's one for NES that is nice but... it doesn't register diagonals, which IMO is pretty stupid. There's another one that -does- register diagonals, it's for the 4 player multi-tap, but it doesn't register button presses from within an emulator. Since people can make stuff like "D-Pad Hero, how hard would it be for someone to just make a nice NES controller test rom that uses decent NES controller graphics, registers diagonals etc?
There are also two Test carts for the SNES, one is a full test cart that tests the electronics and it has a controller test section with really nice "plunk" noises when you press buttons. The other is a Japanese cart that just test the controller, uses the JP / EU button colors, buuuuuut, it has this horrible continuous electronic buzzing sound instead of the nice "plunk" from the US cart. Maybe someone could hack that rom to make it so it's much more pleasant to use.