I know it aint wind, surf, or gravity powered but daaaamn this looks like fun!!!
http://www.iwantoneofthose.com/ProductDetails.aspx?language=en-GB&product=GATGUN&category=toys&CMP=KNC-KEL
Not cheap though!
Al.
...and quite possibly the most pointless item i've ever seen.
...but yet I feel like I want one in my life!