"Black★Rock Shooter" is a character created by huke. She looks like Miku, but she is not associated with Miku or Crypton. Black★Rock Shooter is a 15 year old girl who has cannons on her arms which shoot 20 rock bullets per second.
A VOCALOID producer named ryo was inspired by the character and made a song called "Black★Rock Shooter" using Hatsune Miku. The songs inspired an OVA/movie, and recently an eight episode anime series finished airing. There is also a few manga series and games.
You can read more about Black★Rock Shooter on VOCALOID Wiki or Black★Rock Shooter Wiki. Also, the OVA is very good, I highly recommend it. :) I haven't finished the anime series yet, but it's good so far. (There is also Insane Black★Rock Shooter who appears in the anime, and White★Rock Shooter who is in one of the games.)