You've seen the ads "Run your car on water", "Water4gas", etc... Youtube videos abound of ordinary folks who, with a few parts from the hardware store, have cobbled together fuel cells and put up videos professing upwards of 50-100% increases in fuel economy.
It is certainly normal to approach these claims with a degree of skepticism. After all, if such a device exists, and can be made for under $100, then one might expect that the Hondas or Toyota's of the world would surely add them to a production vehicle.
Well a local news outlet has taken up the challenge to either prove or disprove the claims. You can see the results of their test here: http://www.wptv.com/content/specialreports/story.aspx?content_id=74b15465-2ebb-49e0-acb1-939c4bb13a28