Wise (was Transferwise). Literally made my monthly alimony payments less painful and much cheaper compared to international bank wires. I'm talking about saving 200-300EUR per month on fees. (No I have no affiliation with them.)

Slashdot back in the day.

Digg back in the day.

Hackernews, back .. ... ... .

Java in 1997.

Python and Ruby starting around 2004ish.