Ostinato

Ostinato is an open-source, free and cross-platform packet, traffic generator and analyzer with a friendly GUI.

It aims to be "Wireshark in Reverse" and thus become complementary to Wireshark. With this program, you can open, edit, replay and save PCAP files, modify any field of any protocol (some protocols allow changing packet fields with every packet at run time e.g. changing IP/MAC addresses) and more.