The User hash module allows you to create an individual hash for each user. Use it as a light-weight user identification where you do not want to use the Drupal login credentials. E.g. as an individual API Key for reading insensitive content.
Visotto Easy Menu is a restaurant menu management tool built with Drupal. Restaurant staff can create menu cards and display them on the restaurant's website. If a menu card is updated in Drupal, staff can synchronize the menu card on their website by simply pressing a button. The Drupal application then calls the website which in turn connects to Drupal's REST API to retrieve the updated data.
To avoid requests from unauthorized sources, we developed the user hash module. The user hash is checked upon request both in Drupal and on the website. If the hash doesn't match, access to the data is denied.