Neo4j is released on AGPL. Their subscriptions allow you to pay to legally (IANAL so anyone want to step in here?) circumvent the AGPL and buy support
Price list: http://neotechnology.com/price-list
/peter
"Neo4j is released under a dual free software/commercial license model (which basically means that it’s open source under the AGPLv3 but if you’re interested in using it with closed source software, then you must buy a commercial license)." (http://neo4j.org)
Also, we have an AGPLv3 logo on our web page, there's a LICENSE.txt in our release tarballs, we mention it at the end of every presentation we give, and Neo4j is tagged with AGPL on sites like Freshmeat (http://freshmeat.net/projects/neo4j).
Please let me know how we can make it easier to find!
-EE
Neo4j really is a great tool for maintaining and searching graph data. I use it from Jruby and Clojure in embedded mode.
There are other open source graph databases with more liberal licenses if that is what you need.
I like to take Ext.js as an example they where a front-runner in the JS toolkit space early on and to many looked like they had it in the bag. So much so that one day they switched their licensing to a GPL / commercial model. Overnight it seemed they went from the heir apparent to a footnote in the JS toolkit space.
Licensing has a lot to do with technical selection when it comes to application building blocks, I see Neo4J as a different but in the end similar building block and therefore the licensing is extremely important to me. I think they have a good piece of technology, that due to licensing will most likely leave it in the niche market, therefore I chose to not invest my time in it and learn one of the more liberally licensed alternatives, which since that time have gained momentum. On a purely technical level, I would have chose Neo4J so the moral of the story is that licensing plays an equal part in technical selection.
To be clear, I am not telling anyone how to license their hard work, just offering some insight into how some developers chose the technology to invest their time in.