The microwave oven.

According to this article http://www.vat19.com/brain-candy/accidental-inventions-microwave.cfm, shortly after World War 2, Percy Spencer, an employee at Raytheon, walked past a microwave emitter which was being developed for military purposes, and noticed the candy in his pocket melting. Apparently other Raytheon employees had noticed this phenomenon but they had all ignored it. Raytheon produced the first commercial microwave oven in 1946.