Why is 4chan down today?

4chan recently launched anti-piracy efforts, hiring a firm to look into sites that illegally share pirated movies and music.

In retaliation, 4chan members launched cyber attacks against the Motion Picture Association of America and the Recording Industry Association of America. The cyber attacks resulted in down time for 4chan, as well as for the MPAA and RIAA on Sunday, September 19, 2010.