"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!