mbed TLS (formerly PolarSSL) is an SSL implementation written from scratch designed for use in embedded applications and systems. The API was designed to make sense (unlike some other implementations I could mention) and the source code is written with readability in mind. Written in C as portably as possible. Modules are designed to be as loosely coupled as is feasible. opensource by default, but they do ask you to purchase commercial licenses if appropriate. Has a not-insignificant list of commercial and government users.