The professional-development leaderboard sourced from 540 real comments on a 524-point Hacker News thread. Ranked by how many distinct people actually named each thing — not by marketing.
We fetched the full comment tree for Ask HN: What is the best money you have spent on professional development? (Nov 2020, 524 points, 540 comments) via the HN Algolia API. Every comment was matched against a curated lexicon of well-known PD resources; citations = the number of distinct comments that name the resource. Ties break alphabetically. Prices are public-knowledge estimates (label: est.) and change over time — always verify before buying. Upvote counts are not available per-comment from Algolia, so we rank on citation frequency, which is the honest signal we actually have.