It depends on what kind of traffic you want to generate. If you would like to just send some custom packets, I would recommend Scapy.
The usage page of the documentation describes best, what you can do with it. It works basically on windows too, but there are some problems with wifi interfaces at the moment. That's why I would recommend to run it on Linux. Scapy has also support for Cisco protocols like CDP, OSPF, DTP and EIGRP for example, but they are not part of the official release until now. You'll have to download those extensions from the ticket system.