I know of precisely zero such companies, and I can't imagine how such a company would even have customers in the first place when Terraform already has a $0 pricetag.

I could maybe understand this for Consul or Vault, since those are actual hostable services that could probably be resold - but I don't know of anyone reselling those, either.

Maybe Gruntwork.io, makers of Terragrunt, I think?

I always just stuck to vanilla Terraform but recall coming across them at some point.

I wouldn't call them a competitor by any stretch; more like a partner. A Terragrunt codebase just wraps Terraform, with the same exact backends (including Hashicorp's SaaS offerings) and the same exact providers and everything.

How long before HashiCorp integrate Gruntwork’s “solution” into their own Terraform codebase I wonder?

Pulumi, scalr, env0, spacelift, digger, controlmonkey, terrateam, terrakube

Dozens of them out there I wonder with great interest which one of these got hashicorp worries about their wallet enough to make this move. Could it be env0 raising 42M earlier this year?