Official Junxion site.
JunXion is a USB -> MIDI data routing application for Mac OS X. It enables you to define the translation of each key or joystick action into a specific MIDI event, that can then be routed to any software through a virtual MIDI port, or sent to an external MIDI interface.
Cost: $29
System: Macintosh-only. For the PC, check out VMIDIJoy.