Low Orbit Ion Cannon (LOIC) is an open source network 
  stress testing and denial-of-service attack application
  [...]
  LOIC performs a denial-of-service (DoS) attack (or when 
  used by multiple individuals, a DDoS attack) on a target 
  site by flooding the server with TCP packets or UDP 
  packets with the intention of disrupting the service of a 
  particular host. People have used LOIC to join voluntary 
  botnets.