FreeBSD on Google Compute Engine
FreeBSD is now supported by the service, but images are not provided by Google. Paul Rashidi has posted detailed steps for creating a FreeBSD image.
Later in the same thread, Dave Cottlehuber shares a tool he created to make installation easier.
For RHEL and SUSE, there is a signup link in the Premium Operating Systems documentation.
update: On June 9th, we published a video walking through the details of How to build a custom image for Compute Engine.