Tor
Tor (The Onion Router) is a free, open-source software and network that enables anonymous communication by directing internet traffic through a worldwide volunteer overlay network of relays. It encrypts and routes data through multiple layers (like an onion) to conceal a user's location and usage from surveillance and traffic analysis. Tor is commonly used to access the dark web via .onion sites, but also serves as a privacy tool for general browsing, journalism, and activism.
Developers should learn Tor when building privacy-focused applications, conducting security research, or working in environments where anonymity is critical, such as whistleblowing platforms or censorship-circumvention tools. It's essential for accessing hidden services (.onion sites) and protecting user data from tracking, making it valuable for projects involving secure communications, digital rights, or threat modeling in cybersecurity.