tha's cool, I did saw this site before but didn't notice they have a video annotation tool, just noticed the player, great thing I'll take a lot of ideas from there now :)
I'm the main guy who makes Soundslice. :) Yes, we launched the video annotation tool in 2012 and shifted focus to the sheet-music version in 2014. Our number one bit of feedback was: "Please add support for standard notation!" -- hence the shift.
Here's a tech talk I gave about that original Soundslice version: http://37signals.com/talks/soundslice