WANem allows for detailed simulation of varying WAN conditions (packet loss, latency and more) over multiple links.
The WANem homepage can be found on Sourceforge here:
You can find the published paper about the development of WANem here. It is a project developed by Tata Consulting Services, and released as Open Source under the GPL v2 license.
There are two main versions available at present. The beta release is 3.0 and the older stable release is 2.3
Both are made available as Knoppix based Live CDs and VMWare virtual appliances.