"Reverse"/creating from existing cloud resources:
Terraformer: https://github.com/GoogleCloudPlatform/terraformer
Aztfy (Azure only): https://github.com/Azure/aztfy
Basic GitOps:
Atlantis - https://www.runatlantis.io/
DRY wrapper: Terragrunt: https://github.com/gruntwork-io/terragrunt
Policy as code: OPA: https://github.com/open-policy-agent/opa
Static code analysis: checkov: https://github.com/bridgecrewio/checkov
tfsec: https://github.com/aquasecurity/tfsec
terrascan: https://github.com/tenable/terrascan
kics: https://github.com/Checkmarx/kics
Cost:
Infracost (estimation): https://github.com/infracost/infracost
Terratag (tagging): https://github.com/env0/terratag (disclaimer, I am CEO at env0)
Documentation:
terraform-docs: https://github.com/terraform-docs/terraform-docs
Do you have other Terraform-related open-source tools that you recommend? Please go ahead and add in the comments!