Oasis Workflow

Oasis Workflow is a powerful plugin to manage the editorial processes on your WordPress site.  A visual drag and drop editor allows you to create customized workflows to send posts through multiple users or groups before reaching a published status.  Each stage of the workflow includes options for acceptance or rejection which will send the post to the appropriate step based on actions.

Advanced functionality in relation to customized emails, due dates, custom statuses and more comes pre-packed or as add-ons.  Read the detailed review.