Demo: http://www.soundslice.com/tabs/5680/bohemian-rhapsody-for-so...
It's cool because:
* The state of the art in guitar tabs is horrible ASCII crap (example: http://tabs.ultimate-guitar.com/e/eagles/hotel_california_ta...). Soundslice is a 1000x improvement.
* It solves a key problem for musicians, which is: when you're learning a new song, you generally listen to a recording of it, and it's a pain to cross-reference the recording with the sheet music/tab.
* It's one of the most advanced HTML5 apps on the web. Almost everything is done in <canvas>, and it has dozens of UI details (http://www.soundslice.com/help/). I did a tech talk about the various JavaScript/HTML5 stuff if you're interested: http://37signals.com/talks/soundslice
* Proudly bootstrapped and made by two people.