Instead of resizing your browser, this bookmarklet provides a single page with previews of the page at each device size.
responsive
Responsive Navigation Patterns
Responsive Design Test Bookmarklet
The Multi-Size Web
Awesome collection of responsive, multi-size Web layout resources, tools and articles.
Shorter media queries
HAndy
Adaptive Images in HTML
via @devinsays A small package of PHP + JS that “Automatically adapts your existing HTML images for mobile devices. No mark-up changes needed. Just drop it in and forget about it.”