Xscreensaver will run several programs to continuously update the screen instead
of going blank. You can specify the programs and their arguments using
resources. You can also add dpms (power saver) options.

Xscreensaver will make use of any installed OpenGL libraries, such as Mesa (also
in the ports collection).

The client-server model employed by xscreensaver make it a superior choice to
xlock, since no separate auto-locking program is needed. It's also extremely
easy to add new programs like xearth, or any other program that can fill the
root window as additional screensavers.
