On November 13, Sun announced that large portions of their Java platform toolkit would be immediately available as open source software with a GPL v2 license, with more components to be released over the next months. This is a significant announcement, with the potential to alter the computing industry over the long term. But given that the announcement is several years later than it should have been, and given that there are some significant holes in what has been released, there is not likely to be much impact in the short term.
For most people, the most important part of the code release is related to Sun's Java Standard Edition (AKA, Java SE), which is the mainstream Java platform that most of us use on a daily basis. In particular, Sun released the HotSpot virtual machine interpreter, which is the engine behind Sun's own Java Runtime Engine (the JRE is the software package that most people download from Sun's Java site). Not only is HotSpot the de facto standard JVM for Java applications, it's also already platform independent, and one of the best JVM platforms around, with an intelligent interpreter that does on-the-fly compilation of the most-used code in an application, which makes it much faster than straight interpreters.
Cumulatively, releasing the HotSpot JVM source code under a GPL license should provide for better and faster interpreters everywhere, possibly to the point where the historical promise of "write once, run anywhere" is actually realized (the derisory variation--"write once, debug everywhere"--has been a running joke for almost as long).
For example, even though Sun has already ported their JRE to a fair number of computing platforms, they do not currently have builds for secondary platforms such as Linux on non-Intel hardware, or common BSD distributions, and so forth. Even on platforms that are supported, the lack of a suitable redistribution license has prevented many vendors from including the Sun JVM with their system (this is especially true for Linux, since most vendors will only bundle GPL licensed code with their kits). With the HotSpot JVM now being available under a GPLv2 license, the developers of those platforms can now port the de facto standard Java interpreter to their platforms, and get high levels of interoperability and performance essentially for free, without needing to develop their own JVMs (which are almost always saddled with some kind of interoperability and/or performance problems), and without having to negotiate complicated redistribution licenses from Sun.