OCLC is offering a new feature in their OpenWorldCat project. Readers can now write their own reviews of books! This is done by means of a wiki backend - you have to sign up for an account to write a review (quick and free). After you look up a title, just click on the Review tab and you're off! Here are their guidelines:
The recommended length is 75 to 300 words. Your comments should focus on the item. The best reviews include not only whether you liked or disliked something, but also why. Feel free to mention other items that you consider similar and how this item rates in comparison to them. Please limit yourself to one review per item....Avoid bias, added content should be factual and written from a neutral point of view. Cite sources: list authoritative references when possible. Ensure information cited is verifiable. Don't infringe on copyrights.
Here's an example. Note: reviews are held for approval, so you won't see your stuff go up right away. Also, if you already are registered with WebJunction, you don't need to re-register - just use that login.