[Android, iOS] Free: Knots 3D @ Google Play & Apple App Store

Knots 3D is free for a limited time. Hasn't been free since December and is a popular time waster.

iOS - Google Play

Used by arborists, fishermen, firefighters, climbers, military and boy and girl scouts the world over, Knots 3D will quickly teach you how to tie even the most difficult knot. With more than 140 knots, Knots 3D will be your go-to reference! Grab some rope and have fun!

via nocure at OzBargain