vocaloid

Basically, Vocaloid is a program with voice banks, that people use to make music. There are different characters associated with the voices, the most popular is Hatsune Miku. But all the songs are made and composed by fans of Vocaloid, including all the ones that play at concerts.
Vocaloid is mostly a thing that's popular in Japan, but it has a niche audience in the West too. There are even rhythm video games that feature a variety of songs and make you push buttons in time to the music--I got one of the games as a birthday present one year--that was my introduction into the world of Vocaloid.

The songs vary by a lot. Here are some of my favorites
Daughter of Evil

Witch

Electric Angel

Wintry Wind

Senbonzakura

Melancholic
[60fps Full] Melancholic メランコリック - Kagamine Rin 鏡音リン DIVA Arcade English lyrics Romaji subtitles
Happy Synthesizer
project mirai deluxe - happy synthesizer (ハッピーシンセサイザ) pv