EDE is small desktop environment built to be responsive, light in resource usage and to have familiar look and feel. It runs on Linux, *BSD, Solaris, Minix, Zaurus and even on XBox.

The Trinity Desktop Environment aims to keep the KDE3.5 computing style alive, as well as polish off any rough edges that were present as of KDE 3.5.10. Along the way, new useful features will be added to keep the environment up-to-date.

The MATE Desktop Environment is the continuation of GNOME 2.

The Open Group - What is the Common Desktop Environment (CDE)?
Common Desktop Environment

CDE - Common Desktop Environment the continuation of the original CDE from the open-source release in 2012.
OpenCDE dead link. Use GitHub - OpenCDE instead (dead, replaced by CDE on SourceForge.net).

Petition to Open Source CDE and Motif
The Common Desktop Enviroment Open Source release - 6 August 2012
The Common Desktop Environment - brief tutorial

A framework, not a desktop environment.

GNUstep
Window Maker is an X11 window manager originally designed to provide integration support for the GNUstep Desktop Environment. In every way possible, it reproduces the elegant look and feel of the NEXTSTEP user interface.

The Art is Long, The Den of Multix (aka grey gandalf) for several GNUstep applications.