The most Popular Linux Desktop, Latest version : 3.0 (Released on April 6, 2011), Other major version : 2.3. it’s free and open source software, developed by GNOME community, developers around this planet. it’s based on GTK framework. Most of the Popular distros like Fedora, Ubuntu (not in 11.04) etc. uses GNOME as their default Window Manager.
Another Popular Desktop (free and released under LGPL), based on Qt framework.
a new desktop, Developed by Canonical, will be the default desktop for upcoming versions of Ubuntu.