Sun's OpenLook Desktop - xview library, olvwm window manager and companion programs.
A fast, unobtrusive and modern cross-platform GUI toolkit. Agar fully exploits hardware acceleration where available, but also supports simple frame-buffer interfaces. Agar is stand-alone and easily portable to new platforms and graphics systems.
An experimental Linux desktop based on Enlightenment.
Xynth is a portable embedded windowing system, client/server interface between display hardware (mouse, keyboard, video displays) and the desktop environment that works on many hardware, including embedded devices (handhelds, set-top boxes, etc.). Move project source to github: https://github.com/alperakcan/xynth
jSeamless is a UI abstraction layer for Java to allow developers to write code for any platform and any environment (Web, Desktop, Mobile, etc.) without having to know ahead of time the deployment platform or environment will be.