A little parametric turbine. The openscad program is quite adjustable. I was not at all scientific about designing this. I just designed it look somewhat good. There are probably many ways to make it more efficient. It spins quite well if you blow into it or hold a balloon to the inlet.You can see a quick video of it here: http://www.flickr.com/photos/53725866@N07/5417888910/Print the 3 parts and assemble them with a piece of 3mm filament as an axle. You may need to clean it up a bit or adjust the clearances in the scad file.There is a variable housing_space that defines how much clearance to add between the wheel and the housing (both on the sides and above and below)It should scale pretty well though I haven't tried any other settings than the ones that I printed.