VMware workstation and teams. You can create all the virtual network segments you want for test networks complete with packet loss and soon I'm told by my sales rep latency and jitter. If you want to connect to your computer you have VM to host only connections, NAT connections to share internet, or full bridged connections.
It's worthy every penny. I've used it along with GNS3 (for cisco) to simulate and test routing and QoS in large networks before deploying them live.