XScreenSaver

XScreenSaver is a free and open-source collection of 240+ screensavers for Unix, macOS, iOS and Android operating systems. It was created by Jamie Zawinski in 1992 and is still maintained by him, with new releases coming out several times a year.
The free software and open-source Unix-like operating systems running the X Window System (such as Linux and FreeBSD) use XScreenSaver almost exclusively. On those systems, there are several packages: one for the screen-saving and locking framework, and two or more for the display modes, divided somewhat arbitrarily.
On Macintosh systems, XScreenSaver works with the built-in macOS screen saver.
On iOS systems, XScreenSaver is a stand-alone app that can run any of the hacks full-screen.
On Android systems, the XScreenSaver display modes work either as normal screen savers (which Android sometimes refers to as "Daydreams") or as live wallpapers.
There is no official version for Microsoft Windows, and the developer discourages anyone from porting it. The author considers Microsoft to be "a company with vicious, predatory, anti-competitive business practices" and says that, as one of the original authors of Netscape Navigator, he holds a "personal grudge" against Microsoft because of its behavior during the First Browser War.
The XScreenSaver daemon is responsible for detecting idle-ness, blanking and locking the screen, and launching the display modes. The display modes (termed "hacks" from the historical usage "display hack") are each stand-alone programs.
This is an important security feature, in that the display modes are sandboxed into a separate process from the screen locking framework. This means that a programming error in one of the graphical display modes cannot compromise the screen locker itself (e.g., a crash in a display mode will not unlock the screen).
It also means that a third-party screen saver can be written in any language or with any graphics library, so long as it is capable of rendering onto an externally provided window.
For historical and portability reasons, the included hacks are all written in ANSI C. About half of them use the X11 API, and about half use the OpenGL 1.3 API.
Rather than forking the code-base and re-writing the hacks to target different platforms, XScreenSaver contains a number of compatibility layers.
In addition to sandboxing the display modes, the XScreenSaver daemon links with as few libraries as possible. In particular, it does not link against GUI frameworks like GTK or KDE, but uses only raw Xlib for rendering the unlock dialog box.
In recent years, some Linux distributions have begun using the gnome-screensaver or kscreensaver screen-blanking frameworks by default instead of the framework included with XScreenSaver. In 2011, gnome-screensaver was forked as both mate-screensaver and cinnamon-screensaver. Earlier versions of these frameworks still depended upon the XScreenSaver collection of screen savers, which is over 90% of the package. However, in 2011, gnome-screensaver version 3 dropped support for screensavers completely, supporting only simple screen blanking, and as of 2018, Linux Mint's cinnamon-screensaver 4.0.8 no longer supports the XScreenSaver hacks.
Those Linux distributions that have replaced XScreenSaver with other screen-locking frameworks have suffered notable security problems. Those other frameworks have a history of security bugs that allow the screen to be un-locked without a password, e.g., by simply holding a key down until the locker crashes.
In 2004, Zawinski had written about the architectural decisions made in XScreenSaver with the goal of avoiding this very class of bug, leading him to quip in 2015, "If you are not running XScreenSaver on Linux, then it is safe to assume that your screen does not lock."
The included hacks are highly varied, ranging from simple 2D psychedelia, to 3D demonstrations of complex mathematical principles, to simulations of other computer systems, to re-creations of artifacts and effects from movies.
Though many of the newer hacks take full advantage of the power of modern computers, the age of the project means that some of the older hacks may look dated to modern eyes, as they were originally written for much less powerful computers.
Examples of hacks include:
Some of the included hacks are very similar to demo effects created by the demoscene:
XScreenSaver was featured in Sleep Mode: The Art of the Screensaver, a gallery exhibition curated by Rafaël Rozendaal at Rotterdam's Het Nieuwe Instituut in 2017.