Protocols are extremely important and Neo4j has been opening up to a whole new world with its addition of the REST API. Now people using any programing language can try out this graph database.
Rob Olmost shows an example of using the Neo4j REST API
 from PHP:
I was trying out Neo4j due to my curiosity of the graph specialization. Although Neo4j is not designed to run stand-alone like a database server there is a sub-project that adds a REST API to allow non-Java applications to make use of Neo4j. Neo4j is pretty simple, you basically have nodes, relationships, and properties on both. That’s about it.
As a side note (and I haven’t checked the code it), I think that integrating any language that runs on top of the Java VM would be possible, so imagine having your traversals in your preferred language like Groovy, Python, Ruby, or even Clojure. Pretty cool, isn’t it?