Although I have several others, I like best the Animated Knots by Grog app on my iphone. It is not free (as the associated website is free), but it does not cost much.
One feature on the phone app that I really like (and the website has it too) is the ability to show the mirror image of the instructions. When I am teaching knots to a group, and I find that one is left-handed, I make sure to show that person the app and recommend the website. These left-handed learners always get really excited to see the instructions in the way that makes more sense for them. Unfortunately most instructional materials are written by and for right-handed knot tyers.