XScreenSaver is the standard screen saver collection shipped on most Linux and Unix systems running the X11 Window System. I released the first version in 1992. In 2006, I ported it to MacOS X 10.4.
On X11 systems, XScreenSaver is two things: it is both a large collection of screen savers; and it is also the framework for blanking and locking the screen.
On MacOS systems, these screen savers work with the usual MacOS screen saving framework (X11 is not required).