KDE (Germany :-)
KDE is the other big Unix Desktop environment. It used to be more mature than Gnome but as both have progressed this is no longer the case. Under the hood KDE is powered by a smart component/parts system called kparts. Konqueror is not just a web browser/file manager combo. It is the central hub for the KDE idea of integration. KDE code is clean/object oriented and easily extensible. Writing KDE applications is a positive experience like no other. A great framework is presented to the programmer who can quickly use well-tested code to build his/her own ideas.
The visual part of KDE which is presented to the user is no match for the technologies that power it. The interface closely follows the desktop metaphor and several times it has been accused of being windows-like. KDE tries to offer its users every possible option that can be changed. The control panel, the overloaded toolbars, the kicker and the icon-cluttered desktop show clearly that KDE follows the tried-and-true user interface of the dominant Operating System in the desktop market.
The latest interesting headlines for KDE are the announcements of the solid and plasma projects. There are no published implementations just a simple description in their respective sites. Solid is described as "a system allowing to seamlessly use devices and networks available for your computer". Plasma is an effort to create a "live" desktop for KDE. Although superkaramba exists for some time now, plasma seems to have greater aspirations for the KDE desktop. Plasma is part of the Appeal initiative which at the moment just states their goals. In the same site one can also find the Oxygen Icon theme which advertises itself as "not being just another icon-theme"!.
Knowing how KDE will transform in the future is difficult. Its developers seem to have reached the state where needed functionality is there, and higher level concepts are examined. KDE is clearly a mature desktop that has solid foundations but lacks the final polish that will give it a distinct identity.
As a side note the KDE developers seems to also spend time re-inventing the wheel. Gimp is already there yet Krita is heavily developed these days. OpenOffice is alive and kicking, yet Koffice reminds us that KDE needs everything to be done the KDE way.
Mezzo (Symphony OS)
The Mezzo Interface (part of Symphony OS) really deserves an article on its own. Mezzo tries to present the user with a revolutionary interface which is clearly aimed at casual computer users. The concepts behind Mezzo are best described in the "principles of a simplified desktop user interface and usability experience" whitepaper (downloadable as PDF) which can be found on the site. Mezzo not only has acknowledged the issues we are discussing so far but even proposes its own respective solutions.
The Mezzo interface does not allow any icons on the desktop. There is not any sort of sidebar/start menu. Instead it makes use of the screen corners to organize documents/programs/devices/trash functions. Most Mezzo windows run full screen and being transparent give a "consumer device" feel to the user. All menus are desktop wide and they shrink automatically to show additional entries (no scrolling/nesting). Application windows cannot move off-screen and when minimized go to the bottom of the desktop transforming into miniature screenshots.
Mezzo proposes other interesting ideas like a simplified file manager, and a universal search function (beagle like). It includes its own implementation of desklets powered by Orchestra which should be used to monitor resources and/or show live Internet information(feeds/weather/maps e.t.c).
Mezzo used to be just a collection of screen mockups, but lately a real version is available as an ISO file (currently in Beta). Mezzo is in no way finished but it is getting there slowly. If you are a programmer do not hesitate to help with its development. Mezzo is one of the few projects available which offer something new and refreshing in the desktop field.