This tip is submitted by reader Zacharie:
switchdesk is the command to switch from KDE to GNOME or viceversa. This command provides a simple method of choosing between the various desktop environments available under Fedora Core, Cent OS and Red Hat Enterprise Linux.
If X Windows is running, switchdesk will bring up a dialog box which allows the user to choose between the available desktops installed on the system.
Task: To switch from GNOME to KDE, use the command
$ switchdesk kde
Task: To switch from KDE to GNOME, use the command
$ switchdesk gnome
Please note that file ~/.Xclients, ~/.Xclients-default stores the currently selected desktop.
A note about other distros/BSD
switchdesk is RedHat and friends only command. If you are using different Linux distribution or FreeBSD, open ~/.xinitrc file and type full path to your desktop manager. For example, to use xfce4 desktop:
$ vi .xinitrc
Append following line (your path may be different use, which command to get exact path):
Save and close the file. Enjoy new desktop.
While login you will see option for different desktops (provided that all of them are installed). Usually this is located below Username / password box or lower left button. Just select appropriate desktop (KDE/XFC4 etc).
Load KDE while running Gnome
You can load KDE while running Gnome desktop (thanks to sweta for pointing it out):
Just open your gnome terminal and type the command:
$ startkde &