It’s no substitute for testing with real devices, but the “device wall” view in this Chrome plug-in is a nifty way of getting an overview of a site’s responsiveness at a glance.
A nice rundown of some of the fun you can have with viewport units.
This is a clever technique by Dave—use viewport units to make a lightweight lightbox.
Some really great CSS tips from Rich on sizing display text for multiple viewports.
A comparison of when to use percentages and when to use vw/vh in your CSS.
A handy little bookmarklet for quickly checking how a site might look at different screen sizes, and you can customise it to use whichever screen sizes you like.
Useful advice from Tim on preparing your responsive site for IE10’s new “snap mode”. Don’t worry: it doesn’t involve adding any proprietary crap …quite the opposite, in fact.
A handy little tool for testing responsive designs by automatically changing your browser viewport size.