Microsoft Java
From Software By Jeff
Microsoft does not have a JDK, but they do provide a Java runtime for use with their OS.
Note that support for the Microsft JVM is scheduled to end in 2007. Evidently at that time no more releases of the JVM will be available from Microsoft.
As we're focusing on using the JDK, it is recommended that you go instead to Sun Java for the version appropriate to your OS and project.
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Additional Resources
Microsoft Java (http://microsoft.com/java)