Third-Party Libraries

2D/3D Games

• Cinder
• openFrameworks
• SFML
• Allegro
• SDL
• OpenGL
• Ogre3D
• Irrlicht
• GLEW: OpenGL function loading
• GLFW: OpenGL window manage
• GLM: header only C++ mathematics library for rendering
• assimp: 3D model loading
• VTK
• Magnum: C++11 and OpenGL/GLES/WebGL graphics engine
• Horde3D

GUI

• Qt
  • qwtplot3d
  • qwt5
• wxWidgets
• GTK+
• nana: GUI programming in modern C++ style (Boost License)

Graphics

• Cairo
• libpng
• ImageMagick
• Haru Free PDF Library
• nux
• pangomm
• gegl
• stb

Multimedia

• SFML (Simple and Fast Multimedia Library)
• SDL (Simple DirectMedia Layer)
• Cinder
• openFrameworks

Video

• crystalhd
• mjpedtools
• libmatroska
• libVLC
• gstreamermm

Audio

• PortAudio
• Synthesis ToolKit
• RtAudio
• libsndfile
• fftw
• sox
• soundtouch
• audiofile: format
• flac: format
• chromaprint: fingerprinting
• libofa: fingerprinting
• libmusicbrainz: fingerprinting
• libkcompactdisk: CD

Database

• sqlite3
• mysql++
• brig

Math

• cblas
• alglib
• ArrayFire High Performance Computation Library
• GNU MP: bignum C++ interface
• NTL: A Library for doing Number Theory
• Dlib - linear algebra tools
• Blitz++: linear algebra
• IT++: linear algebra
• Eigen: linear algebra
• Armadillo: linear algebra

Cryptography

• crypto++
• OpenSSL
• Botan
• gnutls
• TomCrypt

Physics

• Chipmunk
• Bullet
• Box2D

AI

• OpenCV

Utility

• Boost: large collection of generic libraries (Boost License)
• JUCE: An extensive, mature, cross-platform C++ toolkit (GPL License)
• Dlib: networking, threads, graphical interfaces, data structures, linear algebra, machine learning, XML and text parsing, numerical optimization, Bayesian nets, and numerous other tasks
• ncurses
• zlib
• libsourcey: Cross-platform C++11 library for high speed networking and media encoding. HTTP, WebSockets, TURN, STUN, Symple and more..
• OnPosix: C++ library providing several abstractions (e.g., threading, networking, logging, IPC, etc.) on POSIX platforms.
• djvulibre
• SVG++

XML

• libxml2
• Reason: xml, xpath, regex, threads, sockets, http, sql, date-time, streams, encoding and decoding, filesystem, compression
• C++ XML objects
• libxml++
• pugixml
• tinyxml
• tinyxml2
• Xerce

Communication

• curl
• curlpp: C++ wrapper for CURL library
• Folly: Facebook Open-source LibrarY. Library of C++11 components designed with practicality and efficiency in mind.
• cpp-netlib: The C++ Network Library
• C++ REST SDK: asynchronous HTTP client and listener, asynchronous Stream, URI, JSON
• libNavajo: light and powerful http server for web application development (LGPL License)
• Chromium Embedded

GPS

• gpsd

PDF

• HARU
• PoDoFo
• JagPDF

Logging

• Log4cpp: A library of C++ classes for flexible logging to files, syslog, IDSA and other destinations (LGPL)
• spdlog: Super fast, header only, C++ logging library
• plog: Portable and simple log for C++ in less than 1000 lines of code (MPL2)

see also A list of open source C++ libraries on cppreference.com