Basic Utility

• facebook/folly - A library of C++17 components designed with practicality and efficiency in mind.
• microsoft/proxy - Next Generation Polymorphism in C++.
• ngcpp/proxy - Next Generation Polymorphism in C++.

CLI

• p-ranav/argparse - A header-only argument parser for Modern C++.
• p-ranav/indicators - Activity Indicators for Modern C++.

Concurrency

• David-Haim/concurrencpp - Modern concurrency for C++. Tasks, executors, timers and C++20 coroutines to rule them all.
• max0x7ba/atomic_queue - C++14 multiple-producer-multiple-consumer lock-free queues.
• taskflow/taskflow - A general-purpose parallel and heterogeneous task programming system.
• NVIDIA/stdexec - std::execution, the standard C++ framework for asynchronous and parallel programming.

Cryptography

• weidai11/cryptopp - Free C++ class library of cryptographic schemes.

Database

• google/leveldb - A fast key-value storage library that provides an ordered mapping from string keys to string values.
• SRombauts/SQLiteCpp - SQLiteC++ (SQLiteCpp) is a smart and easy to use C++ SQLite3 wrapper.

Data Visualization

• alandefreitas/matplotplusplus - A C++ Graphics Library for Data Visualization.

Debugging

• sharkdp/dbg-macro - A single-header dbg(…) macro for C++.

Diagnostics

• jeremy-rifkin/cpptrace - Simple, portable, and self-contained stacktrace library for C++11 and newer.

Formatting

• fmtlib/fmt - A modern formatting library.

Game Utility

• skypjack/entt - Gaming meets modern C++ - a fast and reliable entity component system (ECS) and much more.
• BehaviorTree/BehaviorTree.CPP - Behavior Trees Library in C++.
• godotengine/godot - Multi-platform 2D and 3D game engine.

GUI

• idea4good/GuiLite - The smallest header-only GUI library(4 KLOC) for all platforms.
• GNOME/gtkmm - The official C++ interface for the popular GUI library GTK+.
• ocornut/imgui - Bloat-free Graphical User interface for C++ with minimal dependencies.
• mikke89/RmlUi - The HTML/CSS User Interface library evolved.
• webui-dev/webui - Use any web browser or WebView as GUI, with modern web technologies in the frontend, all in a lightweight portable library.

Image Processing

• opencv/opencv - Open Source Computer Vision Library.
• PointCloudLibrary/pcl - A standalone, large scale, open project for 2D/3D image and point cloud processing.

Internationalization

• unicode-org/icu - A set of C/C++ libraries providing Unicode and Globalization support.

JSON

• bblanchon/ArduinoJson - Simple and efficient JSON library for Arduino and embedded C++.
• nlohmann/json - JSON for Modern C++.
• simdjson/simdjson - Parsing gigabytes of JSON per second.
• Tencent/rapidjson - A fast JSON parser/generator for C++ with both SAX/DOM style API.

Logging

• gabime/spdlog - Fast C++ logging library.

Machine Learning

• davisking/dlib - A toolkit for making real world machine learning and data analysis applications in C++.
• flashlight/flashlight - A C++ standalone library for machine learning.
• mlpack/mlpack - A fast, header-only C++ machine learning library.
• pytorch/pytorch - The PyTorch C++ frontend is a pure C++ interface to the PyTorch machine learning framework.

Math

• arrayfire/arrayfire - A general-purpose tensor library that simplifies the software development process for the parallel architectures found in CPUs, GPUs, and other hardware acceleration devices.
• dpilger26/NumCpp - C++ implementation of the Python Numpy library .
• g-truc/glm - OpenGL Mathematics.

Networking

• netcan/asyncio - A C++20 library to write concurrent code using the async/await syntax.
• boostorg/asio - A cross-platform C++ library for network and low-level I/O programming.
• boostorg/beast - HTTP and WebSocket built on Boost.Asio in C++11.
• CrowCpp/Crow - A Fast and Easy to use microframework for the web.
• libcpr/cpr - C++ Requests: Curl for People, a spiritual port of Python Requests.
• pocoproject/poco - A Powerful cross-platform C++ libraries for building network- and internet-based applications that run on desktop, server, mobile, IoT, and embedded systems.
• sogou/workflow - C++ Parallel Computing and Asynchronous Networking Framework.
• uNetworking/uWebSockets - Simple, secure & standards compliant web server for the most demanding of applications.

Parsing

• foonathan/lexy - C++ parsing DSL.
• taocpp/PEGTL - Parsing Expression Grammar Template Library.
• yhirose/cpp-peglib - A single file C++ header-only PEG (Parsing Expression Grammars) library.

Physics

• erincatto/Box2D - Box2D is a 2D physics engine for games.
• jrouwe/JoltPhysics - A multi core friendly rigid body physics and collision detection library, written in C++, suitable for games and VR applications.

Reflection

• Neargye/magic_enum - Static reflection for enums (to string, from string, iteration) for modern C++, work with any enum type without any macro or boilerplate code.

Rendering

• raysan5/raylib - A simple and easy-to-use library to enjoy videogames programming.
• libsdl-org/SDL - Simple Directmedia Layer.
• SFML/SFML - Simple and Fast Multimedia Library.
• bkaradzic/bgfx - Cross-platform, graphics API agnostic, "Bring Your Own Engine/Framework" style rendering library.

Scripting

• pocketpy/pocketpy - A header-only Python interpreter for game scripting in Modern C++.
• ThePhD/sol2 - a C++ <-> Lua API wrapper with advanced features and top notch performance.
• herumi/xbyak - A JIT assembler for x86/x64 architectures supporting MMX, SSE (1-4), AVX (1-2, 512), FPU, APX, and AVX10.2.
• libriscv/libriscv - a simple, slim and complete sandbox that is highly embeddable and configurable, specializes in low-latency, low-footprint emulation.

Serialization

• google/flatbuffers - A cross platform serialization library architected for maximum memory efficiency.
• protocolbuffers/protobuf - A language-neutral, platform-neutral, extensible mechanism for serializing structured data.
• USCiLab/cereal - A C++11 library for serialization.
• stephenberry/glaze - Extremely fast, in memory, serialization, reflection, and RPC library for C++. JSON, BEVE, BSON, CBOR, CSV, JSONB, MessagePack, TOML, YAML, EETF.

Testing

• doctest/doctest - The fastest feature-rich C++11/14/17/20/23 single-header testing framework.

TUI

• ArthurSonzogni/FTXUI - A simple cross-platform C++ library for terminal based user interfaces.

Unit Testing

• catchorg/Catch2 - A modern, C++-native, test framework for unit-tests, TDD and BDD - using C++14, C++17 and later.

XML

• leethomason/tinyxml2 - A simple, small, efficient, C++ XML parser that can be easily integrated.

YAML

• jbeder/yaml-cpp - A YAML parser and emitter in C++.