At EFF we spend a lot of time thinking about Street Level Surveillance technologies—the technologies used by police and other authorities to spy on you while you are going about your everyday life—such as automated license plate readers, facial recognition, surveillance camera networks, and cell-site simulators (CSS). Rayhunter is a new open source tool we’ve created that runs off an affordable mobile hotspot that we hope empowers everyone, regardless of technical skill, to help search out CSS around the world.
If you're going to protest soon, consider getting and setting up one of these Rayhunter devices. You can do it for less than $50 and help an open source community collect data on "Stringrays" or fake cell-towers that police use to track your location, especially at protests.