Haml (HTML Abstraction Markup Language) is a layer on top of HTML or XML that's designed to express the structure of documents in a non-repetitive, elegant, and easy way by using indentation rather than closing tags and allowing Ruby to be embedded with ease. It was originally envisioned as a plugin for Ruby on Rails, but it can function as a stand-alone templating engine.
Slim is a template language whose goal is reduce the syntax to the essential parts without becoming cryptic.
A secure, non-evaling end user template engine with aesthetic markup.
Generic interface to multiple Ruby template engines
Inspired by ctemplate, Mustache is a framework-agnostic way to render logic-free views. As ctemplates says, "It emphasizes separating logic from presentation: it is impossible to embed application logic in this template language. Think of Mustache as a replacement for your views. Instead of views consisting of ERB or HAML with random helpers and arbitrary logic, your views are broken into two parts: a Ruby class and an HTML template.