Widely used open-source X desktop projects—such as GNOME, KDE, and Xfce—are collaborating with the freedesktop.org project. In 2006, the project released Portland 1.0 (xdg-utils), a set of common interfaces for desktop environments. However, freedesktop.org is a "collaboration zone" for standards and specifications where users can freely discuss ideas, and not a formal standards organization.
freedesktop.org was formerly known as the X Desktop Group, and the abbreviation "XDG" remains common in their work.
Pixman, is a low-level software library for pixel manipulation, providing features such as image compositing and trapezoid rasterization. Important users of pixman are the cairo graphics library and the X.Org Server
libinput, a library to handle input devices in Wayland compositors and to provide a generic X.Org input driver. It provides device detection, device handling, input device event processing and abstraction to minimize the amount of custom input code compositors need to provide the common set of functionality that users expect
PulseAudio is a sound server frontend meant to provide software mixing, network audio, and per application volume control.
systemd is a comprehensive init framework to start and manage services and sessions meant to replace older init models.
Xft, anti-aliased fonts using the FreeType library, rather than the old X core fonts.
Also, Avahi (a free Zeroconf implementation) started as a fd.o project but has now moved elsewhere.