Pdump is a sniffer written in perl which dumps, greps, monitors, creates, and modifies traffic on a network. It combines features from tcpdump, ngrep, tcptrace, dsniff (and its webspy and urlsnarf), pfilt, macof, and xpy. It understands tcpdump-like syntax and allows easy modifications via a plug-in system. Readme available here.
3fb40f2bd523eecdd833e0166a9a1ce515da621cdc8a19adf3910f19c324c883