Simple Taxonomies plugin
Another good one... it lets you create new taxonomies, basically like tags. Could somehow be integrated into the post-form with a dropdown menu.
Note: after installing this one, at first the permalinks for the taxonomy terms returned a "Page not found". But then I changed basic WordPress permalinks back to default and back to my custom permalink again, and that somehow fixed the taxonomy permalinks, so all is golden
http://wordpress.org/extend/plugins/simple-taxonomies/
developer's page:
http://yoast.com/wordpress/simple-taxonomies/
