Logseq offers Task Management, PDF Annotations, Flashcards, Whiteboards and Knowledge Graphs and more in a single FOSS package. It's licenced under AGPL and written in Clojure + TypeScript.

A Beginner's Guide to Logseq

GitHub Page

Being a open source under AGPL of all things is what makes Logseq stand out. I have personally tried out Obsidian and then Logseq. I will most likely stay with both of these apps at least for now.