Welcome to the OpenJDK BSD Port wiki.
Installing
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.
OpenJDK 6
FreeBSD
There is an OpenJDK 6 package in the FreeBSD Ports collection called openjdk6.
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.
OpenJDK 7
NetBSD
There is an OpenJDK 7 package in the NetBSD package collection called openjdk7.
OpenBSD
There is an OpenJDK 7 package in the OpenBSD packages collection called jdk1.7.
Building from source
Mac OS X
Mac OS X 10.5
(Darwin9Build adapted from http://confluence.concord.org/display/CCTR/Build+OpenJDK+Java+1.7.0+on+Mac+OS+X+10.5)
Mac OS X 10.6
(Darwin10Build adapted from http://www.javarants.com/2009/11/01/building-openjdk-1-7-0-for-mac-os-x-10-6-snow-leopard/)