This is not mine, but I saw it recently and thought it was pretty cool. You can adjust the speed it plays back and have it play an accompaniment to you if you are practicing. Pretty awesome, but there are reviews that say the OCR can make mistakes and you can't edit them.
"Show HN is for something you've made that other people can play with. HN users can try it out, give you feedback, and ask questions in the thread."
If you just "saw it recently and thought it was pretty cool", submit it just like any other link.