nonstd-lite/jthread-lite

Portable C11 threads implementation over POSIX threads and win32 threads.

Lightweight structured concurrency for C++20

Boost.org thread module

A modern, fast, lightweight thread pool library based on C++2x

Minimal C++20 STL replacement library. Simpler, smaller, and in many cases faster.

variant lite - A C++17-like variant, a type-safe union for C++98, C++11 and later in a single-file header-only library.

optional lite - A C++17-like optional, a nullable object for C++98, C++11 and later in a single-file header-only library

Extensions to the C++ standard library. This library implements polyfills to enable library functionality from later versions of C++ to be used in earlier versions. It also implements some nonstandard but useful extras.

Lightweight thread library for C/C++ coroutine (similar to goroutine), for high performance network servers.