CFEngine is both the oldest and the newest of the popular tools for automating site administration. Mark Burgess invented it as a free software project in 1993, and years later, as deployments in the field outgrew its original design he gave it a complete rethink and developed the powerful concept of promise theory to make it modular and maintainable.
I contributed to this article by Andy Oram on CFEngine 3. It talks about the history of CFEngine 3, why it is a powerful tool for system administrators, and what is new in the latest releases of both the commercial and the open-source versions of CFEngine.
(disclaimer: I work for CFEngine AS)