Docker is really great for developing things IMO. I use it in a few ways actually. One thing I've found it really useful for is isolating build slaves in Jenkins (using the docker-cloud plugin in Jenkins).

I also like to use it to create test deployments for debugging or evaluating things, for example it's a lot easier to run Hadoop in pseudo-distributed mode inside a Docker container with host networking, than it is to fiddle with running it in a VM and either getting NAT or DNS working just right, or installing it locally. With the Docker container, if anything goes awry, it's just so easy to get back to initial state by killing the container and starting again.

As for Vagrant, I like it a lot too, but for different reasons. You can define a set of actions that is a lot closer to installing whatever it is you are developing, instead of baking everything together like you do with Docker, which can be desirable. I have used it in the past for creating virtualized cluster environments for integration testing of distributed systems. I think so far I use the VirtualBox provider, but I'm thinking of re-working some of my past uses of it that don't strictly require a VM to use the Docker provider.