libposix is an impementation of the core functionality of all Unix systems. It is a full, cross-platform implementation of the POSIX 2008 standard. It is meant to replace existing implementations of a Unix system's core libraries. It is an exact implementation of POSIX 2008 and nothing else (no extensions, no previous POSIX versions). However, it works well with possible extensions to the core system functionallity (for example, GNU or BSD).
libquickmail is a library intended to give C/C++ developers a quick and easy way to send email from their applications. It supports multiple To/Cc/Bcc recipients and multiple MIME attachments without size limitation. It uses libcurl for SMTP communication, but a light version is also included without the libcurl dependancy. The library can also be used to just create a multipart MIME message body. It also comes with a command line application for sending mail (with optional attachments).
librouteros is a library that can be used to communicate with devices running RouterOS, a Linux-based operating system by MikroTik. A generic and low-level interface is provided to issue arbitrary commands to the device and receive replies. For some commands, high-level abstractions are provided to simplify communication. The library is thread-safe and reentrant-safe.
libsas is a Python library to take the pain out of using SaaS APIs. It provides an abstraction layer on top of various APIs, taking care of constructing the URLs, serializing parameters, and authentication. You just call Python methods and receive Python objects. It's like an ORM for SaaS.