EFF is no longer working on pcapdiff, but development continues with the tpcat project.
Pcapdiff is a tool developed by the EFF to compare two packet captures and identify potentially forged, dropped, or mangled packets. Two technically-inclined friends can set up packet captures (e.g. tcpdump or Wireshark) on their own computers and produce network traffic between their two computers over the Internet. Later, they can run pcapdiff on the two packet capture files to identify suspicious packets for further investigation. See Detecting packet injection: a guide to observing packet spoofing by ISPs and EFF's Test Your ISP Project for more background.
Pcapdiff 0.1 is written in Python, is run from the command line, and requires the pcapy Python library. It should run on any OS where those two things are available. This is an early release of the software; more features and bug fixes are expected in the future.