Worm attack generation tools

Hi,:)
Does anyone know any tools which can be used to simulate attack traffic (especially traffic pattern of worm attacks)? It is for the purpose of testing IDSs. I've looked at PACKIT and Netcat, but they can't generate "simultaneous" connections which is required for generating worm spreading behaviour...(or are there any ways to use PACKIT or Netcat to generate simultaneous connections?)

Many thanks!:)