Lots of places have linked to the GlucoBoy story, but it's certainly worth noting. The father of a young diabetic lad noticed that the kid loved his GameBoy but neglected his blood glucose meter, so invented a plug-in device in order to do blood readings with the GameBoy. What stands out about this invention (aside from its "street finds its own uses for things" shine) is the reminder of how widespread general purpose computers are, even in the guise of dedicated-purpose devices.
Alex noted the question of what a $100 computer would look like earlier, and it's easy to fall into the trap of thinking that it must run on an Intel-type chip, or have the standard peripheral interfaces. A GameBoy is pretty close to a $100 computer right now; well over 100 million units have been sold worldwide. What would it take for other non-game modules to come out? Where's the PollutionBoy (with air quality reader)?
I want a PollutionBoy! more to the point, though - what would it take to create such a thing and distribute it to grassroots enviro groups? and then, perhaps, provide a collation or statistical analysis service/program for those who were willing to share their data? (sort of a SETI-at-home for the global ecosystem...)