Terragrunt
Thin wrapper around Terraform — Gruntwork, 2016. Addresses DRY problems: share backend config, provider config and variables across many environments without copypaste.
terragrunt.hcl files specifying source (Terraform module), inputs and dependencies. terragrunt run-all apply runs many modules in topological order. Supports remote state, lock files, output passing between modules.
Sweet spot: large infrastructure monorepos with 50+ env × component combinations. Competitor: Terraform Cloud Workspaces, Atlantis, plain terraform with thin helper scripts. Many switch to OpenTofu + native stacks (2024+) but Terragrunt often remains.