Since version 2.2.0 Neo4j comes with an high performance import tool out of the box that takes many ideas of this one, but is way more scalable across CPUs and has little memory requirements.
The only cases that are not covered are repeated imports in existing stores and population of manual indexes. Please consider the built-in and officially supported tool first, before falling back onto this one.
The simplest invocation is /path/to/neo4j/bin/neo4j-import --into graph.db --nodes nodes.csv --relationships rels.csv with the header format being similar to this one. For a quick intro check the developer pages.
There is much more to it, please see the Neo4j reference manual.
This software is licensed under the GPLv3 for now. You can ask Neo Technology about a different licensing agreement.
