Pond
Draw a line and it makes music. A browser instrument with no notes, so there's nothing to get wrong.
A lot of people I love are convinced they aren't musical. I've never quite believed them. Pond is a page in your browser. Draw lines on it and it will make music. It will probably sound like something interesting, if not good, and you might like it. Your drawings stay, and you can send one to a friend as a link. There are no notes, so there's nothing to get wrong. Open it full-screen, or play with the embedded version below.