Propellor
Propellor is configured via a git repository, which typically lives in ~/.propellor/ on your development machine. Propellor clones the repository to each host it manages, in a secure way. Properties are defined using Haskell. There is fairly complete API documentation, which includes many built-in properties for dealing with APT and Apache, cron and commands, DNS and Docker, etc. There is no special language as used in puppet, chef, ansible, etc.. just the full power of Haskell. Hopefully that power can be put to good use in making declarative properties that are powerful, nicely idempotent, and easy to adapt to a system's special needs!
