Unix friendly network testbench for FreeBSD?

[youshi10 at u.washington.edu]

Hi,
      I'm looking for a network testbench / simulator to stimulate known networking conditions to test out a component for a product at work.
      I was wondering if there was a network simulator available (preferably open source) that's FreeBSD / Linux compatible which I can simulate as real of a network as possible on a virtual machine / network.
Thanks,
-Garrett