The microwave.
Without it, I would have starved to death.
I read a webcomic that has since gone off the internet, so I'll just have to quote it.
<Guy stands in front of microwave>
The "30 second" button is the greatest invention of all time.
<Guy reads a burrito package, "cook time 2:00" throws it in the microwave and hits the minute button four times. Microwave cooks burrito two minutes.>
Yay, it's done! This button can cook anything!
<Guy reads a TV dinner tray, "cook time 4:15." Guy looks at the package, looks at the microwave, looks at the package, looks at the microwave, and finally throws the TV dinner away because he has no idea how to make a microwave cook the TV dinner for 4:15.>