ntdisp, short for NtD In System Programming, is a system for programming LPC and STM32 (and possibly other) embedded devices. It provides three different frontends, a GObject-based library (for developers), a commandline utility (for computer geeks), and a GTK+3 frontend (for less geeky users).
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.
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.
spuug is a little tool that generates the boilerplate code for GObjects. spuug can be used for "plain" GObjects, but also for more sophisticated objects such as GTK+ widgets. It tries to be a bit smart with the code in spits out. However, understanding of GObjects/GtkWidgets is recommended.
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.