The Piano Guys
These musicians were popular with the LDS crowd for a while, but it wasn't until they started using YouTube that they gained widespread popularity with LDS and non-LDS fans alike.
Consisting of Jon Schmidt, Steven Sharp Nelson, Paul Anderson, and Al van der Beek, these guys specialize in renditions of popular songs and classical music, and their videos are beautiful cinematic masterpieces in their own right (see "Story of My Life" below).
All of their videos have over 750,000 views, and several of them have over 20 million views. Their most popular video is a cover of One Direction's "What Makes You Beautiful," with over 34 million views!
Check out their Facebook page for more.