I didn’t know there was a webcollage screensaver in the xscreensaver package until I was experimenting with it as desktop wallpaper.
There’s a Web-driven version here, and if you can set your page to reload regularly (I use the Tab Mix Plus extension with Iceweasel, and I can set it to refresh a certain number of seconds), you get a curious smorgasbord of random images off the web snapped at interval.
The net effect is an interesting way to surf “passively” — the pictures are linked to their origin, and you can check out the source, if you’re inclined. It’s kind of fun, if you’re bored or not really doing anything.
Warning: The Internet is the Internet, people. So I can guarantee, with almost 100 percent certainty, that at some point or another there will be images which are “inappropriate.” So don’t try it if you’re easily offended.