There's LogSeq, which is a bit of a different model, but is open source and has all the basic functionality that Obsidian has.
I don't know what kind of formatting you're looking for, but most of the apps I'm familiar with are markdown or just plain text. I use Vimwiki in markdown for my notes, but it can also be exported to HTML, which I don't use often.
I don't know what kind of formatting you're looking for, but most of the apps I'm familiar with are markdown or just plain text. I use Vimwiki in markdown for my notes, but it can also be exported to HTML, which I don't use often.