Supercell is a group known for their music done with Vocaloid, or more specifically, Hatsune Miku. They’ve recently done the ending song:“Kimi no Shiranai Monogatari” for the anime series Bakemonogatari. Most of their songs are uploaded on Nicovideo and it’s great. Their songs are so popular that they have a lot of different fans playing their own versions of it.

For anyone interested in their songs, I’d recommend starting with “Black Rock Shooter”, “Melt” and “World is Mine.” There’s a lot more, and countless different variations of each song fan-made and “official”, these are just my favorite ones.