Here's a blog post with some details: https://blog.jms.id.au/2015/07/openpower-firmware-stack/
The BMC firmware we're currently working on at IBM (based off some work by Facebook) is at https://github.com/openbmc/openbmc.
[Disclosure: IBM Power Systems. Opinions my own.]
Here's a blog post with some details: https://blog.jms.id.au/2015/07/openpower-firmware-stack/