LibU is a multiplatform C library that comes under a BSD-style license. It includes many interdependent modules for accomplishing several tasks: memory allocation, networking and URI parsing, string manipulation, debugging, and logging in a very compact way, plus many other miscellaneous tasks. It has a small footprint (about 70KB for the default configuration), it is modular, and it has a multiplatform nature, making it an ideal candidate for embedded systems. KLone is an example of such usage.
NaturePHP is a classes wrapper using PHP's autoload to create a non-intrusive, simple, organized, and extensible library system. It is aimed at intermediate and advanced PHP developers, providing a lot of non-intrusive functionality for Web site and application development. Upcoming development will include internal library management and the ability to download/update libraries from the repository.
libjson is a JSON parser and printer that is fully compliant with the JSON spec. The parser is efficient and interruptible with a simple and flexible API. It also optionally supports extensions like C-style comments or Python/YAML style comments, and for security allows the user to set a limit on the number of nested structures and the parse buffer. The library plugs easily into any model since it doesn't include any object model, and plugs easily in any project's memory management since the memory functions can be redefined.
EJDB is an embedded JSON database engine. It aims to be a fast MongoDB-like NoSQL library that can be embedded into C/C++/Nodejs/Python3/Lua applications. It features collection-level write locking, collection level transactions, string token matching queries, and a Node.js binding.