At JUXT we use Clojure's EDN, bolstered with some tag literals courtesy of our Aero library (described here: https://juxt.pro/blog/posts/aero.html). We use ClojureScript to compile to TF's JSON. EDN allows comments, ignores commas and otherwise is a nicer JSON. Aero allows us to encode multiple environments in a single document, and include ciphertext for secrets. We're pretty happy with the overall result.
I've used Embedded Ruby (ERB), I was also converting YAML to JSON before CloudFormation added native support for YAML templates (Sep 2016). My company's web application was a Ruby/Rails stack so `.yml.erb` syntax was already quite familiar to our team. Pre-processing the template is a simple shell one-liner that you can easily add to your test/deploy scripts:
A lightweight template-preprocessor step adds just enough scripting automation (in a familiar language/environment of your choice) to cut through boilerplate, and avoids imposing yet another domain-specific intermediate abstraction layer on top of the whole stack (e.g., troposphere's Python API, arguably also Terraform's HCL).
cfn-builder[0] is my take on simplifying CloudFormation templates and allowing project teams to manage their own stack. I built it specifically to use in a CD/CI environment so it doesn't use things like input parameters that might change from one run to another. Most variables are stored either in a global namespace (AccountId, IP Addresses, AMIs, etc.) or in separate environment namespaces (Subnet IDs, CidrBlocks, etc.) It even includes several built-in commands to help you maintain your environment, including one to update the global namespace with current AMI information.
The CFN coverage is not as complete as I would like but I've built and managed production workloads with it and it does the job. Since it's built on NodeJS so if you know JSON and a little bit of Mustache it's not hard to understand. Anyone want to help?