Nexus-core-ng is a project mainly written in JAVASCRIPT and JAVA, it's free.
Nexus Core NG (some advancements for core eventually applied to official Nexus Core)
This is the Nexus Core, a mirror of the Sonatype Nexus Core SVN repository.
I used as manual a nice blog entry to create a "one way" mirror of SVN repository:
http://www.fnokd.com/2008/08/20/mirroring-svn-repository-to-github/
So, the setup is similar, we have the following branches:
DO NOT FORK THIS REPO, you will be not able to follow it.
Have fun!
~t~
Original readme follows:
This is the Core codebase of Sonatype Nexus, aka "Nexus OSS".
Project homepage: http://nexus.sonatype.org/
Public source repository: http://svn.sonatype.org/nexus
Issue tracking: https://issues.sonatype.org/browse/NEXUS
Public wiki: https://docs.sonatype.org/display/Nexus/Home