1) Navimec navigation buttons:
I really like this one for frontend navigation but it doesn't make sense in-game where I need insert credit and start buttons.
2) Art Lebedev's Optimus Mini Three:
This is more interesting but also more expensive. Since the keys are actually tiny monitors the images inside the buttons can change. This means I can have up, down and select buttons for the front end and Insert credit, start and ESC buttons for MAME. Also, it will probably look amazing with some nice artwork. The downside is that I need to add a USB hub and, of course, the price...