Ansible
Ansible is a radically simple IT automation system. It handles configuration-management, application deployment, cloud provisioning, ad-hoc task-execution, and multinode orchestration - including trivializing things like zero downtime rolling updates with load balancers.
Read the documentation and more at http://ansible.com/
Many users run straight from the development branch (it's generally fine to do so), but you might also wish to consume a release.
You can find instructions here for a variety of platforms. If you decide to go with the development branch, be sure to run "git submodule update --init --recursive" after doing a checkout.
If you want to download a tarball of a release, go to releases.ansible.com, though most users use yum (using the EPEL instructions linked above), apt (using the PPA instructions linked above), or "pip install ansible".