Wikis may be the best tool yet for an elementary school library because you can control who contributes. Thus, it is possible to have a fifth grade only project on a topic and only allow fifth grade students to contribute. (True, you do not know who is behind the keyboard after log in or who the students choose to share the log in information with.) The pages are reasonably ad free and neat with clear instructions for contributing. The Wiki WorldCat concept could be applied to a site library with students contributing reviews of books they have read.
I also like the potential for group editing. What a great way to compile a class newspaper.