3 interactive screen savers (ok, their games too!)

http://absolutist.com/ssaver/

atomic 3d shooter - This great screen saver will soothe your eyes and
mind during long workhours. Colourful atoms floating in space around
your desktop, getting fused and nuked all the time. You can also
download arcade game and take part in all this fun yourself.

intellitris - You can be just a spectator or a player while this
screensaver is running. It allows you to use keys when you think that
its smart engine does not do quite well.

hextris - Watch this new tetris like screensaver playing on your
display. You can play as well, and try to keep these falling hex blocks
at bay.