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).
radare2 aims to create a complete, portable, multi-architecture, Unix-like toolchain for reverse engineering. It is composed of a hexadecimal editor (radare) with a wrapped I/O layer supporting multiple backends for local/remote files, debugger (OS X, BSD, Linux, W32), stream analyzer, assembler/disassembler (rasm) for x86, ARM, PPC, m68k, Java, MSIL, and SPARC, code analysis modules, and scripting facilities. It also has a bindiffer named radiff, base converter (rax), a shellcode development helper (rasc), a binary information extractor supporting PE, Mach0, ELF, class, etc. named rabin, and a block-based hash utility called rahash. Radare was rewritten as radare2, and the old version is only maintained for bugfixes.
gtkaml is an XML syntax and parser that extends the Vala.Parser and transforms all your tags into a valid Gtk+ UI class. It features a casual XML syntax for describing the way Gtk widgets are laid out in a (new) custom widget you're creating. Code "islands" (written in Vala) are used for widget signal handling and other methods/signals/properties that you're introducing. It doesn't depend on an external library at run-time, and it is much more readable than the usual UI boilerplate.
Epris is a D-Bus service and command line client to listen to music. Unlike xmms2 or mpd, it uses GStreamer and D-Bus. It supports all formats supported by GStreamer, and can play both video and audio. It supports gap-less playback thanks to playbin2. It has an extensible command line interface, a small memory footprint, automatic deactivation, and an easy D-Bus interface for other UIs.
Listaller unifies the way you manage software on your Linux system by providing a user-friendly, application-centered software manager GUI. It also provides a software setup package format (the IPK package format), which works on all Linux distributions, as well as tools to make your application binaries work on every Linux distribution. The project has merged with Autopackage some time ago. One of Listaller's strengths is its close integration with AppStream and PackageKit. This means that you will be able to manage Listaller-installed applications with your favorite package-manager, like GNOME-PackageKit, Apper, or even the Ubuntu Software Center. Listaller is primarily designed to be run on Linux distributions, but it could be ported to *BSD.
DeaDBeeF is an audio player for GNU/Linux and other UNIX-like systems. It is written in C with some plugins in C++. It has minimal dependencies, a native GTK2 GUI, cuesheet support, support for MP3, Ogg, FLAC, and APE, chiptune formats with subtunes, song-length databases, and more. It is very fast and lightweight, and extensible using plugins (DSP, GUI, output, input, etc.). The GUI looks similar to Foobar2000.