It's an airfoil simulator program made in Java. The UI is crude, but you get used to it. My only real complaint is that it doesn't seem to have a lift:drag ratio estimator, but given that it's free I'll give that one a pass.
The site this is on has several more cool little simulators for jet turbines, rocket nozzles, and other aerospaceish things. Check 'em all out: http://www.grc.nasa.gov/WWW/K-12/UndergradProgs/index.htm