Sourcetrail (Google) had been my primary go-to for large multi-MLOC C-code and C++ project like Mozilla Firefox recently for me. I can now insert instrumentation testpoints deep inside Firefox JavaScript JIT engine within two days from zero-knowledge, quicker there on.

- Sourcetrail (GUI/Linux/Windows, closed-net-capable, archived) - https://github.com/CoatiSoftware/Sourcetrail

- SourceInsight (repo/web-server/closed-net) https://www.sourceinsight.com/

- OpenGrok (web server plug-in/Java/closed-net) https://oracle.github.io/opengrok/

- CLion (GUI-based IDE) - By IntelliJ/JetBrains - https://www.jetbrains.com/clion/

- SourceGraph (Web-based) https://about.sourcegraph.com (thanks, gravypod)

- Codesee.io (GitHub/web-based) - https://www.codesee.io/privacy-and-security

For free as in beer, I prefer OpenGrok so I can get more than JetBrains