KMid2 plays MIDI and karaoke files to hardware MIDI devices or software synthesizers. It supports playlists, MIDI mappers, tempo (speed) control, volume and pitch (transpose) controls, and configurable character encoding, font, and color for lyrics. The graphic views include a rhythm view (visual metronome), a channels window with solo/muting controls and instrument selectors, and a piano player window (pianola).
D'Enfent Engine is a library that contains high-level classes to access OpenGL functions. It contains classes that may be very useful in game development: GUI components, sprite classes, and classes to handle mouse/keyboard events. It has a full API reference, as well as lessons to help you start working with the library.
Stem Desktop is a minimalistic Debian desktop for low-end hardware with limited memory (166Mhz / less than 64M). The goal is to find programs that have the smallest possible memory footprint. After installing *.deb packages on top of clean Debian, the desktop is ready to accept graphical logins.
If you're a developer of multiple projects, you'll find yourself needing to switch between multiple different version control systems. After working in one (e.g. SVN) for an hour, you may need to switch to a different one (e.g. git). Getting your fingers to switch is the difficult part, or remembering the right command line flags to use for the given tool. That's where The One Ring comes in: it provides a constant interface to all the version control systems, and your fingers only need to remember a single command regardless of what checkout directory your shell is sitting in.
SIMtrace sniffs communication between a mobile phone and a SIM/USIM card. This can be useful for research and analysis, and for developing mobile phone and SIM toolkit (STK) software. The SIMtrace hardware is also capable of MITM (man in the middle) attacks on the SIM-MS communication.
phpdigikam is a Web interface for browsing your Digikam photos. It allows you to quickly share your Digikam photo collection with navigation by albums or tags. You will need a PHP5-enabled server and a few lines of configuration. Features include slideshow, play movies, and re-use of freedesktop thumbnails.