GDB GNU Debugger
The Unix world's standard debugger, originally written by Richard Stallman in 1986. Steps through programs, sets breakpoints, inspects memory and performs autopsies on core dumps.
The basic commands go surprisingly far: break, run, bt for a backtrace, print, next/step, info locals. Less known but valuable are watchpoints (stop when a variable changes), reverse-step for running backwards, and gdbserver for remote debugging embedded systems over serial or network. Works with C, C++, Rust, Go, Fortran and Ada, reading DWARF symbols from the binary — compile with -g. The interface is off-putting; tui enable gives you a source window, and the pwndbg and GEF plugins are standard in reverse engineering. LLDB is the LLVM project's counterpart and the default on macOS.