Operating Systems: Almost every modern OS, including Linux, Windows, macOS, and even microkernel systems, has its kernel written predominantly in C.
Compilers: The compilers for many higher-level languages (e.g., GCC, Clang) are either written in C or C++ (itself derived from C).
Network Stacks: C dominates the implementation of networking protocols. Browser engines, routers, and switches heavily rely on C for their performance-critical network stacks.
Browsers: Chrome, Firefox, Safari, and Edge all have engines written in C or C++ (e.g., Blink, Gecko, WebKit). These engines drive the rendering of the web.
Database Systems: Many databases (e.g., MySQL, PostgreSQL) rely on C for their core implementations due to its speed and fine-grained control over resources.