So way back in 1998, my second internship was at Andros, a company that made gas analyzers. This one contractor who worked there came in one day with something he got at the hardware store: A fart detector! And it really worked, too. It was an actual methane gas detector (thus the relevance to our work), which he proved by flicking his lighter by the nozzle. The result can be seen here, if you click "Try Me".
Sadly, it looks like they're pretty much out of production. The above flash animation comes from this site. I just emailed them to see if they're still in business and have any.