ettercap-project.org
Ettercap is a long-standing suite for man-in-the-middle attacks on a local network. It uses ARP poisoning to sit quietly between two hosts, then sniffs, filters, and rewrites the traffic passing through, with protocol dissectors that pull credentials and content out of live sessions. Useful in a controlled lab for seeing exactly how a LAN MITM works and for testing whether your detections actually catch ARP spoofing.