I find the following libraries for io.js to be essential to a robust web project in 2015. The JavaScript front end development stack is changing dramatically month by month, but solid, usable tools don't come by very often.
Put everything together with Browserify
Browserify is a godsend. Being able to manage my front end JavaScript with NPM feels like the future. This library makes just about anything possible. Grab the companion Watchify library too to watch for changes to your files.
Get it: npm install browserify watchify --save
Deal with old versions of IE with Modernizr
The shackles of old version of IE still haunt us. Modernizr makes it a bit less painful by letting us target unsupported features with a simple body class.
Get it: modernizr.com
Use Koa for your web serving needs
Koa is a lightweight web server perfect for localhost development. It makes use of ES6 features like promises and yield to return content for HTTP requests, but you don't have to learn about that to use it.
Get it: npm install koa --save
Manage your templates with React
React is a little different from your ordinary HTML templating library. You write your templates in JSX (or with JS functions, as I prefer) in a JavaScript. However, it offers the most performant way to update your HTML through a virtual DOM.
Get it: npm install react --save
Simplify your CSS with Stylus
Stylus makes working with CSS easier than any other CSS preprocessor. Get the nib utility library too, for easy stylesheet resets, browser prefixes, and other useful mixins.
Get it: npm install stylus nib --save