Cairo-dock has a family-likeness with OS X dock, but with more options. It uses cairo to render nice graphics, and Glitz to use hardware acceleration. It's fully configurable and can be a taskbar too. You can easily plug applets into it.

PulseAudio, previously known as Polypaudio, is a sound server for POSIX and WIN32 systems. It is a drop in replacement for the ESD sound server with much better latency, mixing/re-sampling quality and overall architecture.

DConf is a low-level key/value database designed for storing desktop environment settings.

RawTherapee is an advanced program for developing raw photos and for processing non-raw photos. It is non-destructive, makes use of OpenMP, supports all the cameras supported by dcraw and carries out its calculations in a high precision 32bit floating point engine. RawTherapee supports JPEG, PNG, and TIFF as output format for processed photos.

Python, the high-level, interactive object oriented language, includes an extensive class library with lots of goodies for network programming, system administration, sounds and graphics.

Nautilus is the official file manager for the GNOME desktop. It allows one to browse directories, preview files and launch applications associated with them. It is also responsible for handling the icons on the GNOME desktop. It works on local and remote filesystems.