Netflix’s Janitor Monkey provides for a statically defined set of rules in code, many of which are overly specific to Netflix’s use cases. It also utilizes legacy technology for state management in the form of SimpleDB. On the other hand, Custodian allows users to define policies and rules to enforce within a yaml configuration file, and, due to its stateless nature it can be deployed on a laptop, server, or lambda.
Netflix’s Security Monkey is a more interesting tool that provides for some basic audit capabilities. It’s worth investigating, but it’s not clear that there is much overlap between Cloud Custodian and Security Monkey as they are targeted to different use cases.