Like most people I was familiar with Wikipedia - the site to go to for a plain/practical explanations of entries; the place to go to add your own encylopedic entry about your town, your school - knowing that others will see/find your work (and reputedly correct wrong info/assumptions within days).
Obviously wikis are the salvation for book reviews/guides and the like - put up your opinions/thoughts on a book and others read it and add to it - great word-of-mouth. However I believe they need to have sufficient content prior to being launched (2-3 books in a category/list do not a wiki make).
We all collect little pieces of information and all choose to file them away differently (in their head, vertical file, odds slips of paper, etc.). The combination of ease and speed of publishing content on a wiki would allow groups or teams to share their findings and information in 2-way system - adding and obtaining information, especially in a regional/geographically isolated situation. A wiki could be used as the new version of the 'New York Public Library Desk Reference' - a ready reference database for frequently asked questions.