It’s also a tool made mainly for Zettelkasten, but it is offline and local by default. It’s not an outliner like Roam, but rather a free-form text editor.
I feel that Obsidian’s values align more closely with the values of a general HN reader. For example, the files (Zettels?) are plain markdown files, so the portability is much higher than what is the case with Roam (which is online only, and your data is somewhere in a database in a proprietary format).
Another example would be the support for plugins, which are first-class citizens (although the API is yet undocumented) — many of the core features are implemented as plugins and can be turned off.
And there’s a Discord channel where you can discuss with the devs, which are very responsive — so much so that I’m surprised they can rollout new features so quickly (at least one feature update per week, from my limited experience with Obsidian).
(Not affiliated in any way, just a happy user. I copied most of this comment from another comment of mine)
I’m a little surprised that there aren’t any mentions of Obsidian, while there are at least two mentions of Roam. To all Roam lovers, and to all intellectuals in general, I’d recommend you to check out Obsidian  from the makers of Dynalist.
It’s also a tool made mainly for Zettelkasten
Just remember that not everyone is in your little concern-bubble, and one or two explanatory sentences would be very welcome.
It's a text based wiki or outliner (collapsible text) thought a step further, with auto backlinks etc.
Feels to me like a weaker org-mode, online with better cross-links/embeds (something that is indeed uncool in org. things can't live in two places at once)
I'll need to switch from one biiiig file to multiple than though. I think my biggest hindrance is setting up the refile targeting :)