Approval Process Reminders is a simple app to help make sure approvals don't stall. Every night, it'll look through all of the pending approvals in your Salesforce org and send an email to anyone who has pending approvals.
To install this application, click Get it Now from the AppExchange.
When going through this install, you can choose Install for Admins.
After installation is complete, go to the Approval Process Reminders tab and click Schedule.
That's it! Now every night at midnight your users will get an email that summarizes any pending approvals.
Bonus: At end of month or other key times, you can click Run Batch Now to send out an ad-hoc reminder for any pending approvals.
Testing in Sandboxes
Make sure emails are enabled. Setup > Deliverability > All Emails
Make sure there is at least one user with a valid email address
Setup an approval process and make sure they have at least one record that is pending approval for the user with the valid email.
Go to the Approval Process Reminders tab and click Run Batch Now