Mac OS X: SecureHome uses your Mac's built-in microphone to listen to potential intruders and sound an alarm if any noise reaches a user-defined threshold. If an intruder is heard, the app can sound an alarm to try and scare them off and email an alert to your smartphone.All you have to do to set it up is open the app, set the alarm threshold, and enter your email address if you want to get notified via email. The big downside of the email function is that it uses Apple Mail — regardless of your default desktop client — to send the message, but if you have a simple account set up to send mail that's not that big of a deal. You'll also want to have your computer hooked up to speakers to make the alarm reasonably intimidating. It sounded pretty wimpy on my MacBook's speakers.
While SecureHome isn't going to replace a complex security system, it is a good way to implement a simple alarm on the cheap.
SecureHome ($0.99) [Mac App Store]