PicoGUI is a small client-server GUI
I have been working on at http://pgui.sourceforge
.net. It is open source
) and primarily targets Linux
but can be easily ported to other operating systems.
Like X windows, PicoGUI has a server that client applications can connect to using sockets (and in the near future, SHM). But, PicoGUI is much smaller and designed to work well on handheld computers, embedded systems and low-end PCs. To avoid the mass of shared libraries in a modern X system, PicoGUI includes themes, widgets, fonts, icons, and an application manager in the server itself, so all you need to run it is the server and the applications.
Besides my attempts to make it as small as possible, I also want to try something a little different than the traditional WIMP interface. There is no desktop. Yes, there is a bitmap that covers unused portions of the screen, but all applications can be accessed through a launcher a-la PalmOS. Instead of windows, applications inhabit application panels that can be rearranged and resized, but never overlap. This is to reduce user confusion and make PicoGUI more efficient with screen space. Dialog boxes are supported, but only for modal operations like selecting a file, or presenting a message box.
PicoGUI still has a ways to go, but it is already suitable for running some specialized applications. Take a look at the home page (http://pgui.sourceforge.net) if you're interested in helping develop it.
At the time of writing this node the server binary (pgserver) is 58 kilobytes with 1 font, 1 video driver, and no input drivers compiled in (all debugging off).