Network simulators

[Ahsan Ali]

Are there any freeware or opensource Network Simulators available for linux? Something like OPNET, but more affordable because I just want to test out a few proposed network changes before implementing them.   If there are no opensource simulators available, can someone please give me a brief list of the good ones out there and their approximate cost? Or a link to a site with the relavant information on it?   Thank you.   -Ahsan Ali