Emacs org-mode
Discord
I think of all of these tools as "personal hypertext notebooks" - basically taking what is possible in wikis (organizing by means of linking, hypertext) and lowering the barrier to entry (no need to spin up a server, can just download an app and go).
The common thread across these notebooks then is allowing for organizing and exploring by means of bidirectional hyperlinks between "notes": - In Obsidian each linkable note is a markdown file and can be as short or long as you like - in Logseq/Roam and other outliner-style notebooks, you can link "pages", and also individual bullets in the outlines on each page.
In this way, the core functionality of these tools is similar to a wiki, but they do leave out a lot of the collaborative functionality that makes wikis work well (granular versioning and edit histories, talk pages, etc.). So for folks like <@305044217393053697> who are comfortable with wikis already, they add marginal value IMO.
Their technical predecessors in the "personal (vs. collaborative) wiki" space include TiddlyWiki and emacs org-mode (and inherit their technical extensibility: many users create their own extensions of the notebooks' functionality. an example is the Roam Discourse Graph extension that <@824740026575355906> is using).
These tools also tend to trace their idea lineage back to vannevar bush's Memex and ted nelson's Xanadu.