Witness - Inexpensive Surveillance App for OS X and iOS
"If you have $40 and a recent Mac with an iSight camera, then you have the makings of a simple home surveillance system thanks to Witness. Developed by Orbicule, Witness is a surveillance app that turns your Mac's camera into a motion sensor. The application runs in the background on your computer and will begin recording video or photos if motion is detected."
Witness offers some decent capabilities for the price, and includes an iOS application that will allow a remote user to view either photos or video of motion-triggered activity. As usual, the reference article's comments are informative, offering an alternative app with similar functionality for a lot less money, and another (also for less money) that captures video and audio. So, if you have a desktop or laptop with an iSight camera, you can become a double-nought spy with a very minimal investment.
"Not everything that can be counted counts, and not everything that counts can be counted." Albert Einstein