Snowman is an advanced Twine 2 story format designed for people who already know JavaScript and CSS. It was originally created by Chris Klimas and is currently maintained by Dan Cox.
Snowman does not use macros. Instead, it provides template tags, <% ... %>, and a JavaScript API for accessing and manipulating the current story and its passages.
It also includes the JavaScript library jQuery.
The official Snowman documentation has more details.