Khövsgöl is a minimal, highly configurable music player and browser. It can easily handle very large collections of music. It's designed for the GNOME desktop, but can run well in other environments. It works fine as a standalone player, but can also be used to remotely control music daemons such as MPD.
OSCATS (Open-Source Computerized Adaptive Testing System) implements Item Response Theory (IRT) and cognitively diagnostic (latent classification) models and item selection algorithms used in Computerized Adaptive Testing (CAT). OSCATS facilitates the development of CATs and simulations of CATs by providing ready-to-use code for running the CAT item selection and ability/classification estimation in an extensible, modular framework. The library is written in object-oriented C using GObject, and has bindings to Python, Perl, PHP, and Java.
Libdmapsharing is a library you may use to access, share, and control the playback of media content using DMAP (DAAP, DPAP, & DACP). Libdmapsharing also detects audio AirPlay services; coupled with the AirPlay support in PulseAudio or GStreamer, this can allow an application to stream audio to an AirPlay device. The library presently supports Linux and other POSIX-based systems. It is written in C using GObject and libsoup. The DMAP family of protocols are used by products such as Apple iTunes, Apple iPhoto, and the Roku SoundBridge family to share media such as music and photos.
libvfcgi is a library that implements FastCGI for GObject and Vala applications, written in Vala. It aims to fully implement the FastCGI specification and make FastCGI Web application servers accessible to Vala programmers and GObject C programmers. Note that while it is possible to use the library from GObject C, it is designed for use with Vala and may be tedious to use directly in C.
notification-thing is a full-featured implementation of the Desktop Notifications Specification. It is not tied to any specific desktop environment, and is focused on features like rate-limiting and flexible filtering to minimize distraction and noise ratio in a transparent way, without silently dropping anything ever.