Ask HN: What tools do you use to understand your codebase?

In my opinion, the hardest part of software engineering is managing cognitive complexity. I'm curious what solutions folks have found to make this easier?

Some examples of products that tackle this general problem are:

- Codestream: discussions embedded in codebase

- Dendron: better documentation

- Readme: interactive api docs

How do you manage complexity in your code? Are there any tools you wish existed that don't?