Vocaloid is computer software for generating synthesized singing, in Japanese and English (though Japanese Vocaloid is more common).  It is also an entire brand, their "artists" have personalities and character.  There's Vocaloid main, who has the characters/singers Hatsune Miku (most famous Vocaloid), the twins Len and Rin, and a few more (I like the others best, so they're the ones whose names I remember ).  There's also Megpoid, a branch of Vocaloid, which is responsible for the character/singer Megpoid Gumi.  I really like the music here, and it's interesting to listen to.  Vocaloid also does live concerts with fancy special effects and holograms to put the anime characters on stage.

Here's one of my favorites (with a very cool video to go with), by Megpoid Gumi called Bad Apple!! (the exclamation marks don't come from me):


This one, Trick and Treat, is by the twins, and is another favorite of mine (the vocals are kinda bad at the beginning but get better):

The music of the twins is often a lot darker than their personality "IRL" suggests.

And here's Hatsune Miku live performing World is Mine:


If you like them, spend some time on YouTube watching more!

Oh god I love Vocaloid so much

(If you didn't noticed it's Len and Rin in my avatar atm )

That must be a very expensive robot.

I love Bad Apple so much, I made it my screensaver.

I love Bad Apple so much, I made it my screensaver.

That's now my screensaver as well.