OrientDB is an Open Source NoSQL with both the features of Document and Graph DBMSs. It's written in Java and it's amazing fast: can store up to 150,000 records per second on common hardware. OrientDB uses the Apache2 License that means FREE for any usage, even commercial!
To begin look at some Presentations, then read Tutorials to acquire the OrientDB's basic concepts. For any question ask to the Community Group. The official documentation is hosted as WiKi under GitHub. Need a course? Look at the OrientDB Academy.
OrientDB is used in production by big companies and startups like:
OrientDB is FREE for any purpose, but if you're looking for professional support look at the Professional Support.