A PS2 BuzzBuzzers controller set for £4 - basically, four USB handsets that with the right software can be used to create quiz games; you can do fastest to hit the button with a big button, or choice of four with four smaller buttons. It turns out that they input as a 20-button USB joystick, e.g. the first controller is buttons 1-5, the second 6-10, and so forth.
It looks like there's some software around for using this on PCs, not sure about Linux or Mac, which is a bit annoying since I don't have a Windows laptop. So far I'm not having much luck. My idea if possible is to put together a really simple package that returns fastest button press and/or choices made by all four users, keeps score, etc. and make it available to SF conventions etc.
It turns out that these are around fairly cheaply - a fiver for the wired version, 40 quid for the wireless ones - from e.g. Play.com
I'm sure that there are uses for these things, the trouble is that I'm not the person to write the software. Anyone know if there's anything good already available? Really don't want to reinvent the wheel!