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Some operating systems include packages of the BSD port. Some of them use OpenJDK 7, others use a backport of the BSD port to OpenJDK 6.
Darwin/Mac OS X
There is an OpenJDK 6 package in MacPorts called openjdk6. To install it, after installing MacPorts, use sudo port install openjdk6 in the Terminal application.
If you do not see the openjdk6 port, try sudo port sync.
Mac OS X
You will find unofficial packages of OpenJDK for Mac OS X (Snow Leopard) regulary built from sources, and the scripts used to build them on the projet openjdk-osx-build
Building from source
Mac OS X
Mac OS X 10.5 and Mac OS X 10.6, could be adapted from http://www.javarants.com/2009/11/01/building-openjdk-1-7-0-for-mac-os-x-10-6-snow-leopard/)