Artificial Intelligence

• frugally-deep - Header-only library for using Keras models in C++. [MIT]
• oneDNN - An open-source cross-platform performance library for deep learning applications. [Apache] website

BitTorrent

• libtorrent - rasterbar) - An efficient feature complete C++ bittorrent implementation. [BSD]

CLI

• argparse - Argument Parser for Modern C++. [MIT]
• args - A simple header-only C++ argument parser library. [MIT]
• FINAL CUT - Library for creating terminal applications with text-based widgets. [LGPL]
• tabulate - Table Maker for Modern C++. [MIT]

Compression

• Rapidgzip - Gzip Decompression and Random Access for Modern Multi-Core Machines. [Apache-2/MIT]
• zlib-ng - zlib for the "next generation" systems. Drop-In replacement with some serious optimizations. [zlib]

Cryptography

• stduuid - A C++17 cross-platform implementation for UUIDs. [MIT]

CSV

• rapidcsv - An easy-to-use, header-only C++ CSV parser library. [BSD-3-Clause]

Font

• libschrift - A lightweight TrueType font rendering library. [ISC]
• SheenBidi - A sophisticated implementation of Unicode Bidirectional Algorithm. [Apache-2.0]

Frameworks

• ETL - Embedded Template Library. [MIT]
• FunctionalPlus - Functional Programming Library for C++. Write concise and readable C++ code. [MIT]

Graphics

• MESHLIB - An SDK to Supercharge Your 3D Data Processing Efficiency. [Free/Commercial] website

GUI

• imgui - Immediate Mode Graphical User Interface with minimal dependencies. [MIT]

Logging

• lwlog - Very fast synchronous and asynchronous C++17 logging library. [MIT]
• plog - Portable and simple log for C++ in less than 1000 lines of code. [MPL2]

Machine Learning

• FANN - Fast Artificial Neural Network library in C. [LGPL]

Math

• CGAL - Collection of efficient and reliable geometric algorithms. [LGPL&GPL] website

Memory Allocation

• Boehm GC - Conservative garbage collector for C and C++. [similar to X11] website

Miscellaneous

• D++ (DPP) - A lightweight, high performance and scalable C++ library for creating Discord Bots. [Apache2] website
• DynaMix - A library which allows you to compose and modify objects at run time. [MIT]
• Klib - Small and lightweight implementations of common algorithms and data structures. [MIT]
• reproc - A cross-platform (C99/C++11) process library. [MIT]

Networking

• Restinio - A header-only C++14 library that gives you an embedded HTTP/Websocket server. [BSD]
• c-ares - A C library for asynchronous DNS requests. [MIT]
• mailio - mailio is a cross platform C++ library for MIME format and SMTP, POP3 and IMAP protocols. [BSD]
• uriparser - Strictly RFC 3986 compliant URI parsing and handling library. [BSD-3-Clause]

Other Awesome Projects

• c-algorithms - C algorithms library.

Reflection

• Ponder - A C++11 library for reflection. [MIT]
• RTTR - A C++11 library for reflection. [MIT] website

Serialization

• Protocol Puffers - A little, highly templated, and protobuf-compatible serialization/deserialization header-only library written in C++20. [Apache-2.0]

Web Application Framework

• lwan - Experimental, scalable, high performance HTTP server. [GPL2]

Yaml

• LibCYAML - C library for reading and writing YAML. [ISC]