In the event that I ever need to do straight C again: C Template Library for a handful of STL containers from C++, such as std:queue, std:stack, and std:string. Comments on hacker news list similar projects:
Also mentioned: Cosmopolitan C Library which "provides the same portability benefits as high-level languages like Go and Rust, but it doesn't invent a new language." Intended for "people who don't care about desktop GUIs, and just want stdio and sockets."