We've discussed "water as a fuel", so it's time to see cars running on air, compressed air to be precise.
http://www.theaircar.com/
Not sure if they're real, or just blowing some hot air.

Production status

As of November 2006 the Air car is not in production; though they have been said to be going into production "soon" since at least 1998.<sup title="The text in the vicinity of this tag needs citation." class="noprint"></sup> It was, for example, announced to make its public debut in South Africa in 2002, or "within six months" in January 2004