At the TC Disrupt Europe hackathon, there was one team that build a hardware/software hack to crowdsource radioactivity data / warnings. Basically, a Geiger counter attached to your computer that sends readings to a server.

I think it was a really cool hack and one of my favorites, but it didn't go anywhere with the jury (I guess no business model and all :) ).

http://techcrunch.com/2013/10/26/open-radioactivity-warning-...

http://techcrunch.com/video/open-radioactivity-warning-syste... (Video)