The Tenori-On iOS app saved my wallet and probably one of the most used apps on my device (other than "Dungeon Raid", a match-three game with RPG elements). $20 dollars for a software version of a $1,000 instrument? YES PLEASE
I just started and Syntorial is amazing! The teacher is very talented and the App you use to learn (which he built with funding from a kickstarter) is very good. The demo comes with 22 lessons and I am a lot more confident with creating and re-creating sounds by ear in my DAW.
I plan on purchasing it but I'm waiting for the "hey you haven't signed up yet so here's a discount" email to come because their marketing strategy is very familiar.