Mac OS X only: Inspired by previously mentioned TweetMyPC, free application TweetMyMac remote controls your Mac from anywhere you can access Twitter, allowing you to restart and shutdown your computer, take a screenshot, open a torrent, and more.
To get started using TweetMyMac, you’ll need to sign up for a new Twitter account dedicated specifically to controlling your Mac, then follow your main Twitter account from your new dedicated account. (Note: You should only follow that one account from your TweetMyMac Twitter account—unless you want to give random people the ability to remotely control your Mac. You’ll probably also want to protect your updates.)
TweetMyMac is freeware, Mac OS X only. For a Windows alternative, check out previously mentioned TweetMyPC.