This is work in progress!
There are a lot of (Free Software, as well as proprietary) firmware implementations available these days, which can be very confusing. This page tries to give an overview.
This project is dead and was merged into coreboot. Use coreboot instead.
Open Firmware IEEE 1275-1994 implementation. Kernel and some modules are written in C. Large parts of the code base is written in Forth. License: GPL
"The original" Open Firmware IEEE 1275-1994 implementation was open sourced in 2006/2007 by Mitch Bradley. License: BSD