PostgreSQL

• PostGIS: Used in a Ruby on Rails application for mapping
• Performance tuning
• Migrating applications from NoSQL
• Clustering

MySQL

Used directly as well as through ORM layers in Java, Ruby, PHP, Python, and C++

SQLite

Used for embedded and memory-constricted devices as well as in mobile applications

Mnesia and ets

Used to manage routes/bindings in RabbitMQ for custom exchange types to allow for wildcard keyword and geographic/location-based routing.