Mounts in the window to exchange polluted air from inside with clean air from outside, while maintaining the temperature inside. This is like opening the window without changing the temperature inside (over 90% efficiency). Use it in your house or in your recreation vehicle.
Update: I have reoriented to making Through Wall units, which are built entirely into the building. There is a wikifactory page for helping you make the window mount units. However, they were not a very good business proposition.
For details on the Through Wall units, see www.openerv.org. There are some pics on the twitter and below.