Artificial Intelligence

Audio, Sound, Music, Digitized Voice Libraries

BitTorrent

These are tools to automate the building and testing of projects in C.

Console/Terminal User Interface, Command Line Interface

List of C or C++ compilers

C library for reading, creating, and modifying zip archives

Concurrency and Multithreading

Cryptography and Encryption Libraries

Database Libraries, SQL Servers, ODBC Drivers, and Tools

List of C or C++ debuggers

These are libraries of all sorts that are useful for game programming.

Graphic User Interface

open source c++ toolkit for image processing.

List of C or C++ nominal IDEs.

Inter-process communication

C/C++ Libraries for interacting with JSON.

This is a list of resources for learning C programming in general, or something useful relating to C programming.

C/C++ logging libraries

Libraries for Machine Learning.

C/C++ Libraries for Scientific Computing

Useful libraries or tools that don't fit in the categories above or maybe just not categorised yet

A collection of libraries and tools to process multimedia content

A collection of open-source libraries for high level network programming.

A NoSQL (originally referring to "non SQL" or "non relational") database provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases

C++ libraries for serialization.

This contains standard C libraries.

List of tools for improving quality and reducing defects by code analysis

libraries for video processing

Full stack web frameworks.

XML is crap. Really. There are no excuses. XML is nasty to parse for humans, and it's a disaster to parse even for computers. There's just no reason for that horrible crap to exist. - Linus Torvalds