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.
MaKL is a simple and light framework for building multi-platform C/C++ projects, purely based on the Bourne Shell and GNU Make. It is much easier to install and use than other typical building systems, while maintaining the essential functionality. It is ideal for embedded systems due to its cross-compilation, multiplatform toolchaining mechanisms, and minimal external dependencies.
libmsocket is a socket abstraction library in C, which aims to support as many protocol abstractions transparently as possible. Current goals include SSL and Socks5 proxies. At its core, libmsocket makes developing socket applications very easy, even if you're not using any of those abstractions at all. It also ties in to libevent for I/O multiplexing and asynchronous DNS resolution.