Radek Mezulanik

Show the number of pending posts waiting for approval in the admin menu, if any. Also automatically supports custom post types. Sends email notification of posts pending review.

Zedna pending post indicator and notifier

Display number of posts waiting for approval in administration. Send an email about waiting approval, approved/rejected post.

evidentlycube

This plugin allows selecting content types (post, page, custom) that are subject to quality control, where specified 
editors have to approve them before they cna be published.
Features: 
1. Separate configuration for each content type. 
2. Define list of users who can approve content. 
3. Define the minimum number of approvals requires for the content to be publishable.

Publish approval

Disallow publishing content until it is approved by a specified number of people.

Zedna pending post indicator and notifier vs Publish approval