Free / Open Source AWS for Auditing AWS Security

Trusted Advisor:
   console.aws.amazon.com/trustedadvisor/

Consigliere (Multi-account Trusted Advisor):

    https://github.com/newscorpaus/consigliere

Prowler (CIS Security Benchmark): 
    https://github.com/Alfresco/prowler
    http://blog.zorangagic.com/2017/07/aws-cis-security-benchmark-tool-prowler.html
    
Scout2:
    https://github.com/nccgroup/Scout2

    https://nccgroup.github.io/Scout2/

    https://n0where.net/security-auditing-tool-for-aws/

Cloud Custodian - Policy based AWS management using Lambda: 
  https://github.com/capitalone/cloud-custodian

  https://developer.capitalone.com/opensource-projects/cloud-custodian/  

Security Monkey - Netflix :

      https://github.com/Netflix/security_monkey

      https://medium.com/@nagwww/secops-with-security-monkey-2ad26cccd5ec

      http://securitymonkey.readthedocs.io/en/latest/quickstart.html

      http://techblog.netflix.com/2014/06/announcing-security-monkey-aws-security.html

Update 30/7/2018 - Good AWS Security tools:

https://blyx.com/2018/07/18/my-arsenal-of-aws-security-tools/
https://github.com/toniblyx/my-arsenal-of-aws-security-tools