Stress Testing LAN/WAN

I have the need to stress test a LAN and WAN. The primary concern is the WAN which is at most OC-3. The LAN would be an additional bonus if I could do that as well. I am familiar with tools such as those from Spirent and IXIA which are very expensive. I was wondering if someone has had to do this and can recommend some open source tools that would work well. I need to test a few different types of traffic, specifically trying to push traffic into various switch/ router policies to make sure everything is performing as expected. If anyone knows of some software that works well for this I would appreciate letting me know. Thanks, Brian