UserPress is a collaborative media (wiki) platform built for WordPress.
If you install UserPress with WP_DEBUG mode on, you will see a lot of PHP warnings and fatal errors. Then, If you disable debug mode the plugin seems to work fine but ..... you can't see any of the wiki entries in the frontend. I like the project but I think it is not ready to be used in a production enviroment.
Don't install it, this plugin replaced my initial theme with theirs, whole site collapsed, thanks God I did backup 3 hours before I decided to check it out.