XScreenSaver is a screensaver program for Unix-like operating systems running the X Window System and Macintosh computers running Mac OS X. It is maintained by Jamie Zawinski.

The free software and open source Unix-like systems use it almost universally, although distributions which ship GNOME and KDE are beginning to replace it with gnome-screensaver and kscreensaver, respectively.

XScreenSaver is released under the terms of the X11 License.

Hacks

One reason for XScreenSaver's popularity is the ease of adding extra modules, known as "hacks", which are separate programs that draw on the screen when the screensaver is activated. Some popular hacks include:

Atlantis - an OpenGL animation showing whales and dolphins.

BSOD - shows fake fatal error screens from a variety of computer systems, including Microsoft Windows Blue Screen of Death, a Linux kernel panic, a Darwin crash, an Amiga "Guru Meditation" error, a sad Mac and many others.

Apple2 - simulates an Apple II computer, showing a user entering a simple basic program and running it.

Barcode - a number of coloured barcodes scroll across the screen.

Electric Sheep - animated Fractal flames

Flow - a 3D display of strange attractors.

Flying toasters - 3D toasters fly around, inspired by the classic After Dark screensaver.

Gears - an OpenGL animation showing intermeshing gears and planetary gears.

GLMatrix - an OpenGL animation similar to the "digital rain" title sequence seen in the Matrix trilogy.

Penrose - shows the screen being aepisodically tiled with coloured Penrose tiles.

Spotlight - puts a moving spotlight across the desktop in the style of the James Bond film opening sequences.

Sproingies - shows an animation in the style of Q*Bert.

XPlanet - draws planets and other celestial bodies that update in real time.

XMatrix - animations similar to the "digital rain" sequence seen in the Matrix trilogy.

Some of the display hack modules in XScreensaver are very similar to demo effects created by the demoscene:

Boing - based on the 1984 program regarded as the first Amiga demo ever, showing the bouncing red and white ball.

Bumps - an implementation of full-screen 2D bumpmapping

MetaBalls - another common demo effect

Moire2 - moving interference circles similar to those common in older Amiga demos

ShadeBobs - another effect common in older Amiga demos

XFlame - the filter-based fire effect, also known as flame effect

Technically, a module is a program that draws on the virtual root window. Whenever the screensaver is activated, XScreenSaver creates a virtual root window, places it above all other windows, and launch a module. In order for hacks to draw in the preview window of recent versions of XScreenSaver, the identificator of the subwindow where to draw is passed to the hacks via the a command line option. If a hack is started directly, it opens a normal window. Some hacks can be useful too - Apple2 and Phosphor include a terminal emulator.