Logic-less templates.
Available in Ruby, JavaScript, Python, Erlang, PHP, Perl, Objective-C, Java, .NET, Android, C++, Go, Lua, ooc, ActionScript, ColdFusion, Scala, Clojure, Fantom, CoffeeScript, D, and for node.js.
Works great with TextMate, Vim, Emacs, and Coda.
The Manual: mustache(5) and mustache(1)