I wanted a drum machine that I could control with $20 of game hardware, so I built it in Excel. A little weird, but I think acceptably eccentric. But Drumpad has its share of problems--funky implementation, non-standard compliant, and often unacceptably laggy when actually played through a joystick. The problem is that I'm trying to build a poor man's MIDI synth.
Well, if I'd thought about it, I would have actually done it with MIDI. CDM featured command line-based utilities, including a keystroke-to-midi translator and a drum sampler, a few weeks back that would make a fine project base. You can find them here. If you were to use ControlMK to link the joystick to those utilities, it's much more responsive, and it's still free. Plus, there's a lot of other fun hackable tricks you could play. I love the command line.