Skeleton is a small collection of CSS & JS files that can help you rapidly develop sites that look beautiful at any size, be it a 17" laptop screen or an iPhone.

The Fluid Baseline Grid System is an HTML5 & CSS3 development kit that provides a solid foundation to quickly design websites with ease.

AmplifyJS is a set of components designed to solve common web application problems with a simplistic API. Amplify's goal is to simplify all forms of data handling by providing a unified API for various data sources. Amplify's store component handles persistent client-side storage, using standards like localStorage and sessionStorage, but falling back on non-standard implementations for older browsers. Amplify's request adds some additional features to jQuery's ajax method while abstracting away the underlying data source.

A great little standalone PubSub JS lib.