well it's certainly FAR from a "low-resource" screensaver, but far and away my all time favorite (and current) screensaver is electric sheep
. Simply beautiful, and never gets boring or repetitive. And I feel that my computer is keeping herself entertained while I'm away.
But, if simplicity and minimalism is what you're after, might I suggest PowerDimmer
from WhitSoft development. It's Donationware, and quite system-friendly. It uses alphachannel to dim the screen so you can still see what's happening "underneath" even when its dimmed. Quite pleasing.
Oh, and it's only 3kb !