dsniff is a collection of tools for network auditing and penetration testing.

dsniff, filesnarf, mailsnarf, msgsnarf, urlsnarf, and webspy passively monitor
a network for interesting data (passwords, e-mail, files, etc.).

arpspoof, dnsspoof, and macof facilitate the interception of network traffic
normally unavailable to an attacker (e.g, due to layer-2 switching).
sshmitm and webmitm implement active monkey-in-the-middle attacks against
redirected SSH and HTTPS sessions by exploiting weak bindings in ad-hoc PKI.