Ask HN: What is that "I didn't know I needed" tool/item until you saw it here

It could be that I'm just one of the 10,000 some days (https://xkcd.com/1053/) but there has been a few times that I've seen an article on HN and went "Umm, I didn't know I needed that, but it fits into a niche use that I have." My last one was Markdeep in a discussion about markup languages. https://casual-effects.com/markdeep/

Or Picotron (https://news.ycombinator.com/item?id=39786984) (https://www.lexaloffle.com/picotron.php) that looks like a great follow on to Pico-8 that I enjoy messing around with.

How often / what is the HN jewel that you've gotten from here that you didn't know you needed? (I'm making the assumption that you all have great search tool skills so you do find the things you need. )