People who consider themselves fans of My Chemical Romance. It's a relatively new term, emerging after the release of their fourth album, Dander Days: True lives of the fabulous Killjoys. Killjoys are often rebellious, and express their rebellion through music, art, or literature. As such, they're somewhat well-know for being artistically gifted in those and other areas. Sometimes, a killjoy will give him/herself a "killjoy name," which is usually an exciting, unique adjective followed by an equally exotic noun.
Killjoys, make some noise!