Simalam

A simple plugin to manage the responsive image specification for your WordPress images. By default it will add 2x retina support to all of your image sizes.
Adds scottjehl’s Picturefill for a fallback for browsers that don’t currently support the specification.

Simages

A simple plugin to manage the responsive image specification for your WordPress images.

PStevenson

In short, this plugin modifies “the_content()” and the post thumbnail function by re-rendering the images with use of the SRCSET attribute. Other plugins do this for templated images, but not in the content region itself. This plugin will automatically work with any previously uploaded images as well.

Post Script Responsive Images

SRCSET responsive images on wordpress for content images.

Simages vs Post Script Responsive Images