My second brain. The back-linking of notes and the graph view of how all your notes are connected are akin to science fiction but life has been known to be stranger than fiction anyway. Obsidian, killed my 10 year+ relationship with Evernote in 30 minutes.
Obsidian is by far the most valuable product I interact with on a day-to-day. I've been using it since the beginning and the evolution of the community and product is so beautiful. Obsidian is quite literately my second brain until I die.