Traditionally, M3-customers are using relational databases (DB2/SQL-Server) for most of their data-storage. 
Document-based databases are becoming increasingly popular, and in many cases they are superior to relational databases.

Especially the flexibility, speed and search-capabilities they offer, makes them perfect for many "Add-ons" to M3, where you are storing data outside of M3.

Have a look at one of the most popular ones: MongoDB