xscreensaver 4.23-4ubuntu2 (i386 binary) in ubuntu dapper

 The purpose of xscreensaver is to display pretty pictures on your screen
 when it is not in use, in keeping with the philosophy that unattended
 monitors should always be doing something interesting, just like they do
 in the movies.
 .
 The benefit that this program has over the combination of the xlock and
 xautolock programs is the ease with which new graphics hacks can be
 installed: you don't need to recompile this program to add a new display
 mode, you just change some resource settings. Any program which can be
 invoked in such a way that it draws on the root window of the screen can
 now be used as a screensaver without modification. The programs that
 are being run as screensavers don't need to have any special knowledge
 about what it means to be a screensaver.