(The blog was posted 4 hours ago and it's already dropped to the 4th page; people don't like Docker?)
I would really like to see the Docker provider for Vagrant happen. Vagrant is so easy to manage, and it would make Chef/Puppet provisioning easy. It also makes cross-platform and EC2/DigitalOcean deployment easy with their respective providers. (i.e. you can deploy an image to EC2 that then runs Docker containers)