An open-source C++ library developed and used at Facebook. [Apache2]

Abseil C++ Common Libraries. [Apache2]

A cross platform open source toolkit for creative coding in C++. [MIT] website

An advanced, open-source C++ framework for high-performance server applications on modern hardware. [Apache-2.0 License] seastar.io

Electronic Arts Standard Template Library. [BSD]

An all-encompassing C++ class library for developing cross-platform software. [Core-Module: ISC, Rest: GPL2/GPL3/Proprietary] website

A glib-like multi-platform c library. [Apache2] website

A tiny boost library in C++11. [MIT]

Embedded Template Library. [MIT]

Functional Programming Library for C++. Write concise and readable C++ code. [MIT]

A cross-platform DAG framework based on C++ without any 3rd-party. [MIT]

The BDE Development Environment from Bloomberg Labs. [Apache2]

libPhenom is an eventing framework for building high performance and high scalability systems in C. [Apache2]

C++11 evented IO for real-time video streaming and high performance networking applications. [LGPL]

Siv3D (OpenSiv3D) is a C++20 framework for creative coding (2D/3D games, media art, visualizers, and simulators). [MIT] website

Framework for Enterprise Application Development. [Apache2]

a set of C++ platform abstraction libraries for macOS, Windows and Linux. [MIT] website

A multiplatform utility library written in C. [BSD]

Fast, Orthogonal, Open multi-methods. Supersedes Yomm11 [Boost]

Minimal Rust-inspired C++20 STL replacement. [MIT]

A collection of std-like single-header C++ libraries [MIT]

Declarative UI toolkit for C++20. [MPL2]

A free and open source C++ modular multi-purpose cross platform RAD framework. [MIT] website

Simple and yet powerful cross-platform C library providing data structures, algorithms and much more. [MIT]

Minimal headers-only C++ Library. [Boost]

STDCXX, A collection of algorithms, containers, iterators, and other fundamental components. [retired] [Apache2]

Apache Portable Runtime. Another library of cross-platform utility functions. [Apache2]

Adobe Source Libraries provides peer-reviewed and portable C++ source libraries. [MIT]

A community-developed, free and open source library for professional-quality creative coding. [BSD]

A small, header-only library that backports various facilities from more recent C++ Standards to C++03 and later. [MIT]

GLib provides the core application building blocks for libraries and applications written in C. [LGPL]

A C++ library of designs, containing flexible implementations of common design patterns and idioms. [MIT]

A cross platform framework designed to bring the ease of use of Java, .Net, or Python to developers who require the performance and strength of C++. [GPL2]

A set of OO frameworks with all the functionality needed to handle and analyze large amounts of data in a very efficient way. Used at CERN. [LGPL]

An exemplary version of STL. [Free]

Standard Template Library for Extra Large Data Sets. [Boost]

A C++ cross-platform rapid application development framework. [BSD]

A C++ library for developing Windows applications and UI components. [Public]