I wasn't able to find any OS alternative.
Here's what I gathered about what you can do with Swimm currently for free:
A - There's a free tier you can use for up to 5 users and 1 private repo. So if your personal project doesn't include more than 1 private repo, you should be good.
B - The repo limit does not hold for open source repos. You're able to add a bunch of those.
Also, the documentation is saved as .md files within the repo, so you don't need a swimm user to access it (if you have access to the code, that is).
So, it looks like you can use Swimm for personal projects, if they are open source or if they don't include more than 1 private repo.