Valgrind used dynamic binary translation and shadow state to turn ordinary program execution into a heavyweight analysis environment, making memory errors observable without recompiling the target program.
Debugging is more than defect removal: it is an investigative practice in which programmers use failures, traces, experiments, and program structure to learn what a machine actually did.
Richard Stallman's GDB made source-level debugging part of the GNU system and evolved into a portable, community-maintained debugger for many languages, architectures and remote targets.