They also did a surprisingly excellent cover of What Makes You Beautiful ; most of the song was re-created by plucking the strings inside the piano. Which is quite genius.

Also, if you liked ThePianoGuys you'll probably like 2CELLOS. Headbanging awesomeness recreated with cellos. That first link was "Welcome to the Jungle", and this one is ACDC's "Thunderstruck". The music video cracks me up every time. They've also done "Back in Black" and "Smooth Criminal" and "Highway to Hell" and just so many other great classic rock songs, they're just so ridiculously talented.

Finally, I feel like everyone's head of Pentatonix at this point, but in case you haven't here's one of my favorites of theirs.