Web-based Guitar Fretboard Trainer: Learning where the notes are on a guitar fretboard can be a long painful process. There are a load of tuition programs available but I've never seen one I liked, so I used learning about the HTML5 Canvas as an excuse to write one. It remembers your performance in finding notes and gives you more practice on the ones that you're having trouble with.
Web-based Harmonica Note Reference Tool: Which holes on a diatonic harmonica make up the scale or chord you're looking for?
If you like puzzles, try my Sudoku Puzzle Generator.
Conway's Life: A little simulation that really abuses html tables.