Should I Sleep is a very innovative system utility that can prevent your Mac from going to sleep using a variety of sensors. Most of the times, your Mac going to sleep is not an issue, but it can be when you are doing important things like transferring a file, or when your computer is doing work for you.
The application comes with one free sensor, and you can purchase additional ones from the "Sensor Catalogs" screen, which uses In-App Purchases. The included sensor is called "Face detection". It uses your Mac's camera to take a series of pictures. The app looks for the features of a face: mouth, nose, eyes, etc. If it finds a face, it then prevents your Mac from going to sleep. There is a bar that you can adjust in the Preferences window that will determine how many pictures are taken by the app to scan for a face. The more pictures it takes, the more accurate the sensor will be, but it will use more CPU power and memory.
The other sensors include a camera motion sensor, a download monitor, an external display sensor, and a sound activity sensor. Each sensor costs $1.99, or you can purchase them all for $4.99 with free updates for any new sensors that are to be released in the future.