How to make a Big Red Button, complete with Python libusb code
blog.metricfire.com
blog.metricfire.com
The posted solution requires custom software, but the above are more customizable yet require nothing special be installed.
[1] http://www.kickstarter.com/projects/joylabs/makey-makey-an-i...
The MakeyMakey would work but is overkill because it's designed for sensing capacitance rather than a key press.
Actually, if you have any modicum of skill at soldering you can easily use an old keyboard. I turned a GuitarHero guitar I found at Goodwill into a Frets on Fire controller that way.
I've used the V-USB library in a couple of fun projects - it's nothing short of awesome.