Final answer:
In the context of Sitecore xDB, additional databases required are the Collection Database (MongoDB), Reporting, and Session State databases to enable full functionality for analytics and personalization.
Step-by-step explanation:
When running xDB in a Sitecore context, the databases required in addition to the Collection Database (MongoDB) are the Reporting and Session State databases. The Reporting database is used in Sitecore to store aggregated data about interactions and contacts, and is key for reporting and analytics features. The Session State database is used to store session data which can be shared across a Sitecore cluster. It's important to configure these components accurately for xDB to function properly and for the Sitecore experience platform to provide comprehensive analytics and personalization features.