Some of them link to http://dynamicdrive.com/ , which is probably where Chruser / Wetwired got the script (looks like Chruser's handiwork, though)
I think it has something to do with the whole April 1st thing.. but maybe not. It IS messing around with my browser though, when they go offscreen it puts a horizontal scrollbar at the bottom (for obvious reasons), when they come back the scrollbar goes away (as you would expect). So I get a flashing scrollbar.
If it slows your computer down then I guess it isn't as good of a thing as I thought. I am at college now at the library and this doesn't slow it down at all. My computer at home is crappy so it probably will be slowed down.
Basically an HTML filter. You can filter ANYTHING on a page. Frames, tables, anything that's actually present in the HTML source is editible here. I could've changed the little falling white things to smiley faces if I wanted to, just by looking for that "peace.gif" instance and replacing it with a smiley URL.