I'm not sure what are the benefits of supporting many VM providers for a tool that is meant for development (as far as I understand at least). Isn't VirtualBox sufficient? It is open source and runs on Linux, OS X and MS Windows. Perhaps I am missing some core idea.
Then there is veewee[1] which started life as a Vagrant plugin but as of version 0.3.0 it can be used standalone and one can ssh to the boxes using it, just like with Vagrant. If they were to add provisioning it would probably be an alternative to Vagrant feature for feature so maybe we're seeing a bit of competition between these two tools there.
On Ubuntu there is VMBuilder[2] written in Python but it doesn't offer provisioning as far as I know.
Last, as others have mentioned there is libvirt[3] which offers tools for communicating with different VMs but I don't think it offers a tool that matches what Vagrant does 1:1 (most notably the provisioning part).
1. https://github.com/jedi4ever/veewee/blob/master/doc/README-c...
2. https://launchpad.net/vmbuilder
3. http://libvirt.org