Here's a simple collaborative art project on 3jay.com. Alun did a series of shadowy drawings in charcoal and viewers submit their interpretation of what's going on in the images.
Object Relations Technique <www.3jay.com>
Very very simple. It's based on a psychological test. Interesting responses. Add some of your own.
Not quite as simple, but uncomplicated, is "The Bench" collaborative comic strip at http://www.thebench.org/index.php3?strip=536 <www.thebench.org> (actually, that points to one by SITO cgi-guru, Jon Van Oast).