Network simulation software?

Does anybody know of any network simulation software that is freeware or shareware that will not cost an arm and a leg?  I'd appreciate any suggestions, thanks!

[Keefe]

I really do not exactly know what you are looking for but you might be able to find something usefull at this site http://www.topology.org/soft/sim.html

I'm looking for network simulation programs like OpNet and basically any software where I can simulate traffic on a network and do analysis on it.  I checked out the links and it looks like I can use a few over there.  Thanks a lot for the help!