With Code Search, I have:
* Rewritten a CMake build system, which would have been practically impossible without access to real-world examples because of how poorly designed and documented it is;
* Validated machine-generated translations by looking up language strings from projects that used human translators;
* Tracked down bugs in unfamiliar codebases, using symbol-based navigation, without the downtime of fetching a bunch of files and waiting for a language server to process them locally;
* Reviewed how projects were using the APIs of a library I work on to determine whether high-maintenance features were actually used, and whether tricky features were being used correctly or needed redesigning to reduce programmer error
Kudos to the team at GitHub. Genuinely stellar work.