I believe SolarWinds has something called WAN Killer, which does just that, I've used it a few times successfully, but I'm not sure if the demo version includes that.

I'm not sure if this will work, but If the network you want to flood is a test network, then you might be able to create broadcast storm using a few hubs and creating a loop. I dont think doing this will give you the results that you are looking for, but it should generate traffic (depending on how many hosts you have on that network)

^^^^Thinking outside the box^^^^