Ask HN: What are your favorite obscure but brilliant C/C++ libraries?

Libpostal - street address parsing and normalisation library that relies on downloadable NLP model.

The judy array library is amazing for hardcore high performance with large memory resident data structures in C, and now there's no more patent on it. Somebody on here took issue when I asked about it once. Disclaimer: no, you don't need judy arrays and no they aren't better than your favorite thing.

https://judy.sourceforge.net/

I mostly use basic libraries, but I do like the ideas coming from Abseil and GSL. A longer list is available here: https://en.cppreference.com/w/cpp/links/libs