Empire is great. That team really knows what they are doing too.

CloudFoundry and Deis are other open source PaaS projects.

Swarm, Mesos and Kubernetes are other open source schedulers on top of which you can build a deployment system.

Cloud Foundry is my own pet favourite, since I work for the company doing the biggest share of engineering. But people sometimes overlook that Red Hat have repackaged Kubernetes into OpenShift Origin.

Take a look at tsuru.io.