Limit Post Creation Per Day is a plugin by wordpresslivro that allows admin to limit the number of posts per day each user can create.
For example you can limit authors to eight post per day, contributors to five posts per day, etc. If some user exceed his limits, he will not able to create news posts. The Plugin has others options, it allows you set no limits for special users just entering they IDs on the plugin options page.
Download and info: http://wordpress.org/extend/plugins/limit-post-creation-per-day/