arno.org has a great article on how to reduce the CPU usage in your air apps. The meat of it is essentially not using ENTER_FRAME (duh), and more interestingly dropping the framerate to once every 10 seconds (0.1) once the air application goes in the background (minimized/hidden).
The ROTCAPTCHA system requires users to adjust randomly rotated images to their upright position. This is a task most people will be familiar with given the state of early digital cameras, etc.
This system has many advantages, such as being language independent and support many input sources.