[papercrane]

OpenJDK was never an independent clone. OpenJDK was started by Sun and was all of the JVM code they could relicense. Contributions to it were done either by Sun themselves, or by companies and individuals that signed a contributor agreement. This has continued under Oracle. Notably this includes Redhat, IBM, Microsoft, Azul, Apple and SAP. Originally the code didn't have enough components to produce a useful JVM, and Redhat put a lot of effort into producing builds that integrated with IcedTea to produce a fully Free Software JVM. Now though the OpenJDK has all of the components needed to produce a JVM.

OpenJDK is Oracle's reference implementation of Java, and provides a GPL+Classpath exception licensed build on their website. Additionally, Oracle provides a build a commercially supported build from their website. The commercial builds from Oracle have included other additions, but over the years the amount of closed sourced add-ons has decreased.

Other vendors provide builds of the OpenJDK, most include additional code, although for the most part the changes are minimal. All of these builds are based on the OpenJDK, and adhere to the GPL+Classpath exception.

There is also Azul Platform Prime/Zing. Which is there own JVM with an LLVM-based JIT, I'm not sure if they're using any OpenJDK code, but if so I assume they have a commercial agreement with Oracle.