IcedTea

There is indeed a close working relationship between IcedTea and OpenJDK. And IcedTea has always promoted itself as not being an actual fork of OpenJDK, just a way for hackers to work together without any legal/political/technical constraints.

http://icedtea.classpath.org/ closely tracks OpenJDK, makes sure the code base can be bootstrapped with a free toolchain (GCC/GCJ/GNU Classpath), has a framework to make it possible for people to package things in GNU/Linux distributions, plugs some small holes in OpenJDK, so one can be sure to not need any proprietary blobs still left in OpenJDK/ClosedJDK, integrates with other free software projects like Cacao, VisualVM, Rhino, etc. Is a testing ground of portability of HotSpot through Zero and Shark. And adds missing pieces like Java Web Start (JNLP) and a browser plugin (Applet) support. For those that wish to sign the SCA code is contributed to OpenJDK (if existing code licenses allow assigning all rights to Oracle of course).