FROONT seems to be worth playing with.

A jQuery plugin for creating slick, app look-alike sliding menus for you mobile website with only one line of javascript.

Piotr Walczyszyn created nice tool in Responsive Inspector (Chrome Web Store link).

fast and lightweight (3KB) javascript utility that detects browser and pixel ratio, allowing you to serve conditional JavaScript and CSS files. Rebuilt from it’s jQuery predecessor, it’s now 50% faster.

Nice overview how to code for high pixel density (160 PPI and up) displays.

Via @daeltar

In terms of images, the goal of web app developers is to serve the best quality images as efficiently as possible. This article will cover some useful techniques for doing this today and in the near future.

HTML5 Rocks Tutorials

Good list, you may find some other resources in my Zootool bookmarks. Also, try searching “responsive tutorial” etc.

This piece of work by Zurb may come in handy.