What are some good open source code explorer tools?

Are there any good open source code explorers for reading large code bases? I'm currently using Magento2 and I'm getting by using their Github and Github's search feature as documentation.

I thought maybe the Github desktop browser would allow a similar type of experience as Github and allow you to browse cloned repos that are saved locally; but it doesn't offer that feature.

I'm on Windows. But, I do have a Linux machine so suggestions for both/either platforms welcomed 🙂