GNet is a network library. It is object-oriented, written in C, and built upon GLib. It is intended to be easy to use and port. It supports abstract Internet addresses, TCP, UDP, IP Multicast, IPv6, asynchronous DNS lookups, SHA, MD5, Base64, URIs, and SOCKS. It comes with documentation and examples.
Libgda is a (relatively small) database access library. It provides a wrapper like ODBC but with more features for accessing several database engines, a general data model for use with CSV or XML files, a metadata extractor that retrieves information about database objects in a common way, and a SQL console application (like mysql, psql or sqlite3 consoles).
GNUstep is a cross-platform, object oriented environment composed of frameworks, tools, and servers (daemons). It is very similar to the Cocoa frameworks from Apple, and tries to maintain compatibility with Cocoa wherever it is desired and possible. The frameworks provide classes for containers, distributed objects, object archiving, file management, text system, font management, image composition, WYSIWYG PostScript graphics, and more.
GSMLIB is a library to access GSM mobile phones through GSM modems. Features include: modification of phonebooks stored in the mobile phone or on the SIM card, reading and writing of SMS messages stored in the mobile phone, sending and reception of SMS messages. Additionally, some simple command line programs are provided to use these features.
GTK, which stands for the Gimp ToolKit, is a library for creating graphical user interfaces. It is designed to be small and efficient, but still flexible enough to allow the programmer freedom in the interfaces created. GTK provides some unique features over standard widget libraries.
Hoard is a scalable memory allocator (malloc replacement) for multithreaded applications. Hoard can dramatically improve your application's performance on multiprocessor machines. No changes to your source are necessary; just link it in. Hoard scales linearly up to at least 14 processors. The supported platforms include Linux, Solaris, Mac OS X, and Windows NT/2000/XP/64.
interpcom is a command interpreter library in C that can be used to build scientific applications. It is very easy to add new commands. The library contains an expression evaluator that can be used to parse the arguments of the commands. It is possible to define "objects" which are arrays of numbers having a name, and structures. It is possible to use threads (several programs running simultaneously).