In addition to using the screensaver hacks as screensavers, they can be started with various command line options as toys for your desktop (e.g. using the -root switch). On slower systems, screensaver hacks may be more successfully run in their own window than in the root window because they take less CPU to refresh. Use the -demo option to try out different screensavers.
See also:
http://packages.debian.org/xscreensaver-gnome---- http://bugs.debian.org/xscreensaver-gnome