Sousveillance is surveillance by the common person

relatively passive / undetectable monitoring

• passive radar
  • KrakenSDR open-source project [code] [hackaday article]
• automatic number-plate recognition
  • OpenALPR (an open-source solution); a tutorial for building ANPR on top of Python+OpenCV

active monitoring that's still fairly covert

• covert listening devices, AKA bugs
• laser microphones