xkcd
xkcd.com
Randall Munroe's webcomic covering science, mathematics, programming, linguistics, and culture — with 2,900+ strips ranging from genuinely instructive to profoundly absurd, updated three times a week.
Why it is useful
Deceptively educational — Munroe has a physics background and the scientific content is unusually accurate and interesting. The 'What If?' series answers absurd physics questions with real calculations. Many strips have become cultural reference points in tech (the password security comic, the 'someone is wrong on the internet' strip). xkcd.explain.org has annotated explanations for every strip when the joke needs unpacking.