Available screensavers at $XDG_CONFIG_DIRS/screensaver as desktop files.
You can execute screensavers under TryExec of the desktop entries to see how they look like.
XDG utils package
Lock screen and activate screensaver:
xdg-screesaver activate
Seems to reset to the current desktop's default if any is recognized such as gnome-screensaver.
Seems to fail is another screensaver such as gnome-screensaver can run.
Seems that it was rewritten and screensaver change is not implemented as of 07/2013: http://askubuntu.com/questions/64086/how-can-i-change-or-install-screensavers