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There's the free one, iAlertU.

Undercover is a more comprehensive pay service that does the same thing but also actively tries to track down your Mac when it gets on a network.
 
WOOHOO! Undercover is exactly the one I was looking for - with so many 'My macbook was stolen' threads, I figure a few bucks to HELP keep it safe is worth it.

I don't think you'll have a problem if you just don't leave your macbook sitting out in the open for anyone to take. 😉
 
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