Interactive Music Theory Cheat Sheet
I just started learning the piano again, and using modern apps (such as Simply Piano) makes it actually fun. Ten years ago, I couldn't bear through reading sheet music, making mistakes and got discouraged. But now, learning to play is like playing Guitar Hero. You get instant feedback, guidance on what and how to play. It's great. We'll see how long it stays "fun", but atm I'm making progress every day.