Website: http://phet.colorado.edu/
Free iPad app: http://itunes.apple.com/app/tinkerbox-hd/id415722219?mt=8&uo=4&at=10l6SU
Free Mac program: http://sketchup.google.com/
Free Windows program: http://sketchup.google.com/
Free iPhone app: https://itunes.apple.com/us/app/wind-tunnel-lite/id387302636?mt=8&uo=4&at=10l6SU
Free iPad app: https://itunes.apple.com/us/app/wind-tunnel-lite/id387302636?mt=8&uo=4&at=10l6SU
Paid iPhone app: https://itunes.apple.com/us/app/wind-tunnel/id381971296?mt=8&uo=4&at=10l6SU
Paid iPad app: https://itunes.apple.com/us/app/wind-tunnel/id381971296?mt=8&uo=4&at=10l6SU
Free iPad app: https://itunes.apple.com/us/app/autodesk-digital-steam-applied/id605304785?mt=8&uo=4&at=10l6SU