There's a pedal from Vidami that works with a number of online resources including YouTube and Vimeo, and a number of specific guitar websites like Guitar Zoom and JustinGuitar. You need to use either Chrome or Safari browsers, as the unit integrates with a special browser plug in.
I don't think it works with desktop media players, which is a bummer. I have the best success with downloading Griff's lessons and running them locally off my desktop. Don't know whether Griff's online stuff would work with it either.