What is the greatest invention of our time?

What's the greatest physical invention in human history? An article in Intelligent Life magazine asked this question and came to the conclusion that it's the internet. Broadening the discussion, a debate on the Today programme asked what man's greatest mental invention would be : a toss-up between maths and democracy, apparently. Can the internet really be considered the greatest physical invention, or has something like the washing machine actually made more difference to our everyday lives? What do you think? And what about the greatest ideas of all time?