If your Mac has ever dimmed its screen, or worse, gone to sleep while you were using it, then you may like the Should I Sleep app from Mr. Milk's Technology Dreams.
Should I Sleep
Should I Sleep uses available Mac sensors to detect whether you're present and working on your Mac. If you are, then Should I Sleep prevents your Mac from taking a snooze.
The free version of Should I Sleep can detect whether a person is facing the Mac. Additional sensors are available as in-app purchases. You can add the ability to prevent sleep by detecting camera motion, a download in progress, if an external display is connected, if a processor-intensive task is being run, or if there is any sound activity occurring near the Mac. Each sensor is $.99, or you can purchase them all for $2.99.
You can custom configure each sensor, and test the sensor's settings as you configure them.
Should I Sleep is free. A full set of usage sensors is $2.99.
See other software choices from Tom's Mac Software Picks.