I blogged earlier about the growth of open source virtual appliances, which now includes outfits like Jumpbox who create value-added appliances for popular open source packages. Now comes rBuilder, for those with a bit more do-it-yourself spirit.
rBuilder (from rPath) uses what they call a "recipe-based definition" model for building appliances. Each of the components that goes into a given appliance is referred to by a specific name and revision number, so that when you generate the appliance from its components, you get the same appliance each time and dependencies between components don't break or get changed unwittingly. The components are tracked and delivered through the Conary packaging system (also used by Foresight Linux), which can track package changes on a file level rather than just the package level.
The finished appliance can be "printed" to file formats recognized by any of the popular virtual machine applications out there: VMware, Microsoft Virtual Server, Xen, Parallels, etc. As with Jumpbox, you can either use the end result on your own, or buy a subscription and get updates and revisions delivered automatically.
One thing that did catch my attention is the amalgam of licenses used in the rBuilder Appliance. rBuilder itself also isn't open source, at least as far as I can tell. But I've suspected for a while now that we're going to see more of this kind of amalgamating of open and proprietary.