Itheon Network Emulator 4.0
Emulating a LAN or WAN, with all its foibles and delays, INE 4.0 lets you test your applications under real-world conditions - without letting them loose on the real world.
Lets you test your client-server apps and related systems in a lab environment.
Wide variety of different constraints you can apply to traffic.
Daisy-chaining of virtual interfaces is a nice touch.
Not cheap, but at least the cost includes the host hardware as well as the software.
Some trivial differences between the user guide and the built-in SuSE screens (eg. for setting the IP address of the control interface).