Stove Knob covers:
http://www.amazon.com/Stove-Knob-Cov...734548?ie=UTF8
They don't seem to get very good reviews, but for $5 it's worth a shot.
Tonight I turned on my lower oven to cook something only to find out about 15 minutes later that my wife had left a wooden cutting board in there to dry. Luckily I caught it before it ignited.