Amazon has Knots 3D as their free app of the day (7/18...for about 6 more hours). It's regularly $1.99 I found it to be very informative with 3D "videos" of tying about 70 different knots, hitches, etc.
http://www.amazon.com/mobile-apps/b?...ode=2350149011
Hopefully this link works...if not, do a Google search on Amazon AppStore, and it should be featured on the front page.
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