The Knots 3D app is free today (18/03/15) in the Amazon app store - usually £1.09 I think. It's a well-designed app with animations for loads of knots.
http://www.amazon.co.uk/Nynix-LLC-Knots-3D/dp/B00701L5XS
http://www.amazon.co.uk/Nynix-LLC-Knots-3D/dp/B00701L5XS