The Kaffe Virtual Machine
Kaffe is a clean room implementation of the Java virtual machine, plus the associated class libraries needed to provide a Java runtime environment. The Kaffe virtual machine is free software, licensed under the terms of the GNU General Public License.
Kaffe is not an officially licensed version of the Java virtual machine. In fact, it contains no Sun/Oracle source code at all, and was developed without even looking at the Sun/Oracle source code. It is legal -- but Oracle controls the Java trademark, and has never endorsed Kaffe, so technically, Kaffe is not Java.
Kaffe has moved to GitHub! Go there to get the latest downloads, source code, and documentation.