Air conditioning from soft drink bottles
Believe it or not, you can use softdrink bottles to make a kind of air conditioner. When air is forced through a smaller aperture (hole), heat is given off and the air is cooled down in a process known as compression (use a bike pump and feel the heat near the valve and the coolness of the air for a quick demonstration).
These videos explains more...
Here is another project that uses the same principle, but in this case, a fan forces air through the bottles. This raises two prickly questions. The first is, we're still using electricity to work the fan. Can we really say that this is sustainable (and is there a way to make it sustainable)? Secondly, the bottles may reduce the temperature of the air flowing through them by a few degrees, but heat is being produced in the bottles (the heat taken out of the air) and the motor in the fan actually creates heat. Is this a pointless exercise?
Here is another variation on this theme (this one is slightly different, but it is still about employing passive means to cool air).