Set up a Twitter account for your PC then install TweetMyPC on your Windows-based computer. Now, just send a tweet to that account you created and you can shutdown, restart or log off your PC via twitter.

A few notes from my experience in testing this:

• Do not use your regular twitter account. Set you a separate account just for this or it won’t work.
• TweetMyPC checks for new tweets once a minute. Because of this do not set it up then immediately leave your PC. Leave it running for a few minutes to make sure no errors are reported once you start it the first time. I did just that and came back a hour latter to 60+ errors on my screen that I had to clear.