My own addons for my 11ty-based sites. YMMV
To include this in your 11ty project, enable git submodules with git submodule init then clone this repository into the src/_11ty/ path with git submodule add https://github.com/JohnVidler/11ty-addons.git src/_11ty
Most functions can be included directly into your .eleventy.config.js by handing the eleventyConfig object into their exported functions:
...
module.exports = function(eleventyConfig) {
...
require( './src/_11ty/shortcodes/gravatar.js' )( eleventyConfig );
require( './src/_11ty/shortcodes/require.js' )( eleventyConfig );
require( './src/_11ty/shortcodes/mermaid.js' )( eleventyConfig );
...
};