WorkFlowy is an infinite-level outliner released in 2010 by a US team, known for Zoom focus and minimalist text. The free plan offers limited nodes and Pro costs about $4.99-9.99 per month, making it a pioneer of outline-based thinking.
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.
Logseq is an open-source local-first note-taking app launched in 2021 by a US team, built around outlining and bidirectional links; notes are stored as Markdown files locally with PDF annotation and whiteboard support, a popular choice for building personal knowledge bases.