Chef
Configuration management in Ruby — Opscode, 2009 (now Progress Software). "Cookbooks" contain "recipes" that describe the desired system state.
Pull model: a Chef client on each node fetches config from the Chef server and runs a convergence loop until idempotent. Pure Ruby DSL makes recipes flexible but requires Ruby knowledge. InSpec is its compliance-testing tool.
Lost momentum to Ansible (push model, agentless, no Ruby) and Terraform (IaC). Lives on in finance and regulated industries where compliance accountings are already written in Chef.