TweetMyMac [1] is possibly one of the most bizarre, but then instantly obvious, ideas that I've seen in a while. The software uses Twitter as a messaging mechanism that enables you to control your Mac remotely.
The folks at Twitter have long made available access to the system via an API (application programming interface) used by developers outside the company to create Web, mobile, and desktop applications. TweetMyMac is software that runs under Mac OS X and receives messages from a Twitter account you set up specifically as a control channel.
The concept is simple. Direct messages in Twitter are received only by the party to whom the messages are addressed. Thus, if you set up an account for a Mac you want to control, you have a direct conduit to it. Your Mac's account must then be set to follow one or more Twitter accounts from which you want to send commands; only accounts being followed may send the Mac direct messages to be executed as remote commands.
The set of commands is small but interesting. You can remotely restart, logout, sleep, or shut down a Mac; launch and quit applications; retrieve its IP address (perhaps useful in case of theft if TweetMyMac remains active); pull a picture from the built-in camera or grab a screenshot (hmm, also useful for a stolen Mac) which is sent back to you via Twitter; and lock the screen (errr...same comment); among other commands. Perhaps most useful is the capability (disabled by default, due to its power) to execute Unix shell commands.
The software, by TheMacBox [2], is free, although donations are encouraged.
[1]: http://themacbox.co.uk/tweetmymac/
[2]: http://themacbox.co.uk/about-us/