I use CFEngine to manage tens of thousands of Linux hosts. I've used their enterprise edition with their agent, which is really outstanding. I've also run it at the same scale agentless, using their community edition. I have used it for one-time configuration at system installation, and have run it every 5 minutes on every host in our plant. The speed of execution and the clarity of the language are second to none, and when I was using their enterprise edition, I found their support and engineering services to be top-notch. I've been using it for 15 years or so, across several companies. While other configuration management systems get the job done, I haven't used any as straightforward and scalable as CFEngine. Many thanks, Mark Burgess!