There are two options for running screensavers under X Windows: xset and xscreensaver. Both of them can be run from xdm while nobody is logged in.

xset is simpler, but it only works if your operating system understands your video card sufficiently well. To test whether it works for your system, log in, type "xset s 5" and wait 5 seconds - if it works, then the screen will go blank (or display an X pattern if you had run "xset s noblank" before). You can also use the "dpms" option to configure DPMS features i.e., to control when your monitor will switch into a power-saving mode.

xscreensaver is a daemon which runs in the background, and is controlled by the foreground programs xscreensaver-demo (graphical) and xscreensaver-command (text-based). When nobody is logged in, xscreensaver will typically run as root, and change to running as nobody before displaying anything on the screen. Unfortunately this can cause problems with xauth security - on our network xscreensaver is set up to run as xuser rather than root. When somebody is logged in, xscreensaver should run as that user and access their configuration files.