Obsidian is the local-first knowledge management app from Ottawa, Canada, released in 2020, storing notes as plain-text Markdown with bidirectional links, graph view and 2000+ community plugins; core features are free forever.
Reflect is an AI-powered, end-to-end encrypted notes app combining Daily Notes, bidirectional links, and built-in AI summaries and Q&A at about $10/month, an Obsidian-style choice for privacy-conscious users.
Capacities is an object-based personal knowledge management tool from a German team, launched in 2019, organizing notes, people, projects, and ideas as objects; the free plan offers unlimited objects, and Pro unlocks calendar and backlink views for about €10/month.
Roam Research is a personal knowledge management tool from San Francisco, founded in 2018, that pioneered block references and bidirectional links. Built around Daily Notes, outlining and a graph view, it costs about $15 per month and defines the "Roam-like" PKM paradigm.