Bento, a project by the same company that makes Chef, is a Packer-based project for building base boxes. Unlike http://vagrantbox.es, Bento just uses base operating systems from the manufacturers with no pre-installed software. All the boxes are hosted on S3 in the following format:
https://opscode-vm-bento.s3.amazonaws.com/vagrant/PROVIDER/opscode_OS-VERSION_chef-provisionerless.box
For example, Ubuntu 13.04 lives here for VirtualBox:
https://opscode-vm-bento.s3.amazonaws.com/vagrant/virtualbox/opscode_ubuntu-13.04_chef-provisionerless.box
And the VMWare equivalent box lives:
https://opscode-vm-bento.s3.amazonaws.com/vagrant/vmware/opscode_ubuntu-12.04_chef-provisionerless.box
For a full list of Bento boxes, see the VirtualBox and VMWare sections of the Bento README on GitHub