XScreenSaver is a screen saver for X11. (Duh.)
Written by Jamie Zawinski, famous for things like UNIX Netscape...
XScreenSaver is vastly better than the previous try, xlock, in that in XScreenSaver the different screen savers are separate executables and in order to add a screensaver to XScreenSaver it isn't necessary to recompile the thing.
Also, the controlling program is a bit more powerful, too, and it also has a decent demo mode. Different screen savers can be selected using a GUI tool. Muchly amazing.
One of my favorite xscreensaver hacks is bsod.