Docker Swarm (built into Docker 1.2) is in the same econiche as Kubernetes, but is not as mature. It is still catching up, and probably more on par with pre-1.0 Kubernetes feature-wise. Kubernetes is what Docker will be when it grows up.

Note: I don't work for Docker or on the Kubernetes team. I have put things into production with Docker, Docker Compose, AWS ECS, and Kubernetes 1.1 on AWS for the past two years and have built tools on top of K8S.

Kubernetes, docker swarm mode, mesos/marathon and ecs are all in the same space.