Lol Power Puddle is great!
I always liked the bubble wallpaper on my brother's Captivate because the bubbles lit up when touched.
My personal favorite, though, is the Nexus version that is included with CM7. Simply because you can choose from several backgrounds and colors for the 'neurons'. I always used a backround where the textured cubes were very small and not distracting, and made the colors all different shades of blue. Looks much nicer than the standartd Nexus wallpaper imo. Plus I like the touch interactivity with it too
Just need to get it on my GSII and all will be good.
Samsung Galaxy S II (SGH-I777) running SHOstock3 v3.0
HTC Aria running CM7.2 Liberty with Drowningchild's kernel