The Nexus Indexer module used by the Nexus Maven Repository manager as well as the M2Eclipse plugin has been released under the EPL v1.0 license.
You can get the source here.
The snapshots are stored in the repository at http://repository.sonatype.org/content/repositories/snapshots and you will need to add this to your Maven settings.xml to build it.
The full Nexus Source will be added to the same repository soon...we are wrapping up beta-3 and will make the move at that time.
Enjoy!