In 12 hours, 3 minutes and well, 40 seconds from the time of writing this, Shutdown Day Begins - see http://www.shutdownday.org/ for details.
To summarise (or rather, to steal their summary):
It is obvious that many people would find life extremely difficult without computers, maybe even impossible. If they disappeared for just one day, would we be able to cope?
Be a part of one of the biggest global experiments ever to take place on the Internet. The idea behind the experiment is to find out how many people can go without a computer for one whole day, and what will happen if we all participate!
Shutdown your computer on this day and find out! Can you survive for 24 hours without your computer?
See the website for details, counts, comments from people on what they plan to do instead of using their computer, and more.