If you are a regular at YouTube, you have no doubt seen Pentatonix sing practically everything. This talented a capella group was originally discovered on NBC's "The Sing-Off" during the show's third season. They went on to win the competition and from there they have put out a few albums and have become quite the internet sensation. If you doubt that, just take a listed to a few items on their YouTube channel . Meanwhile, enjoy the video below, in honor of Valentine's Day.