The a cappella group Pentatonix won season three of 'The Sing Off' in 2011 and now they are huge on YouTube. Their newest video is called "The Evolution of Michael Jackson". They do 25 different songs in the order they were released. 

Pentatonix is often abbreviated as PTX. I first heard of them back in 2013 when I heard one of their Christmas songs!

Here is a link to their social media sites:

PTX Official website
Follow them on Twitter: @ptxofficial
Like them on Facebook: Pentatonix
Subscribe to their YouTube Channel: PTXOfficial