<picture> and srcset/sizes
Since the first half of 2014 there has finally been agreement on how the web community and browser developers might take a common approach implementing a solution. This is thanks to the Responsive Images Community Group and a large body of supporters in the broader web community.
Following painstaking work the <picture> element, and srcset and sizes attrbutes are finally supported by the major browser manufacturers, with the Picturefill polyfill, giving solid support for 'legacy' browsers, we should now be seriously considering using these new functions in all of our projects.
A recording is available of Ben's talk on Practical Responsive Images from 'Digital Henley'.