[tillk]

I don't consider myself to be magical, but anyway — since you were asking.

We primarily use MySQL and CouchDB (BigCouch). Our user records (accounts and payment) are stored the RDBMS, while the data the users created is in BigCouch. We enjoy the schemaless nature, durability and scalability of BigCouch a lot.

Depending on your definition of "database system", we also have a hefty Solr index (for search), some Redis (no persistence only to connect systems/services via pubsub) and Memcache (cache).

[jasonmccay]

Yep ... in interacting with many developers, setups like this are almost commonplace.