mcal stands for Modular Calendar Access Library. libmcal is a C library for accessing calendars. It's written to be very modular, with pluggable drivers. One of the primary drivers it handles is ICAP, allowing access to remote calendar stores. libmcal is the calendar equivalent of the IMAP c-client library.
libAstronomy provides a set of astronomical routines to ease the calculation of ephemerides. It includes mathematical functions to compensate various astronomical phenomena like precession, nutation, etc. The getconstellation routine finds the constellation for a given coordinate. It also calculates the positions of the sun and moon in two different precise ways.
The keeper library provides a simple-to-use C interface for reading and writing configuration data. The data is stored in text files in a hierarchical system. The concept is not dissimilar to that of the Windows registry, but the interface is much cleaner. The library comes with both a command line program and a GUI program for examining and modifying the keeper database.
jUSB contains Java source code for a USB development library with some basic USB tools. It is built on top of the Linux "usbfs" facility. It has both an API (for applications) and an SPI (used by Linux and RMI providers). The API is demonstrated by three bundled utilities or sample applications, and by http://jPhoto.sourceforge.net (for digital camera access).