Open JDK - Open Source implementation of Java
Open JDK is the open source implementation of Java platform. It has loads of sub projects like JDK6, JDK7, HotSpot, Graphics, Swing, Web etc.
JDK 6 and JDK 7 projects are based on the specification of Java SE. OpenJDK 6 sources have passed the Java SE 6 Test Compatibility Kit (TCK) suite. IcedTea builds the source from Open JDK and adds 64 bit plugin and Java web start etc.
Open JDK is actively developed and it is compatible with Java specification.