Best for the rabbit hole.
For long-form curiosity sessions:
Wikipedia Random — the canonical entry. Every click opens a different article. Two hours can disappear into Byzantine succession crises or obscure beetles or 19th-century opera composers.
Atlas Obscura — the world's strangest places, foods, and customs, all properly written by editors who care. Started as a niche curio blog in 2009, now publishes daily.
Pudding Cool — interactive data essays. Each piece is a 10-15 minute scroll that you remember weeks later. "The Pop Lyric Frequency Visualizer." "30 Years of American Anxieties." "How Couples Meet."
Public Domain Review — curated archive of public-domain images, essays, and weird historical material. Run by historians. Updates regularly.
Letters of Note — Shaun Usher's archive of correspondence "deserving a wider audience." Newsletter format now but the web archive is still browseable.
For more in this register, the Time Wasters and Hidden Gems categories are the deep dive.