freedesktop.org (xdg) is open source and discussion project working on interoperability and shared technology for X Window System desktop desktops. The most famous X desktops are GNOME and KDE, but developers working on any Linux/UNIX GUI technology are welcome to participate. They create open standards that project can comply with.

FreeSoftwareFoundation

shared-mime-info - package contain descriptions of many filetypes

freedesktop.org homepage

NETWM - freedesktop.org spec

Development of a xdg compliant menu for Puppy thread

Freedesktop.org (Wikipedia)