SiteSell.com offers this type of feature with their Site Build It! product. SBI! provides a complete set of tools to research, build, and maintain a site. It also includes the option to add their Content 2.0 feature. It's an interactive feature where visitors can submit topics and other visitors can comment on the submissions.
Content 2.0 is really easy to use and you don't have to have any skill in databases or design... It does all the backend stuff for you.