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.
YAJL (Yet Another JSON Library) is a small event-driven (SAX-style) JSON parser written in ANSI C, and a small validating JSON generator. It's highly portable, data representation independent, fast, generates verbose error messages including context of where the error occurs in the input text, can parse JSON data incrementally off a stream, and is tiny.
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.