Membership systems are no panacea, and they won’t stop a person who is committed to disrupting your site. But they do offer an important stepping stone to connect a website’s community to a real person, and that person to their actions.
The smartypants over at GM are running a contest to "make your own" commercial for the gas-guzzling Tahoe SUV. People are giving them just what they deserve.
Lazybase allows anyone to design, create and share a database of whatever they like.
Tag friends and associates to create contextual relationships
A free online service (or, as we like to call it, a Playground) for building and using social applications.
A collection of open-content textbooks that anyone can edit.