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.

WordPress.org

George Stephanis

Helen Hou-Sandi

Andrew Nacin

Kelly Choyce-Dwan

Brandon Lavigne

Chris Coyier

Joe McGill

wilto

Jasper de Groot

tevko

Bringing automatic default responsive images to WordPress.
This plugin works by including all available image sizes for each image upload. Whenever WordPress outputs the image through the media uploader, or whenever a featured image is generated, those sizes will be included in the image tag via the srcset attribute.
Important notes
<ul>
<li>As of WordPress 4.4, images are responsive by default. If you are on WordPress 4.4 or plan to update, you will not need to install this plugin.</li>
</ul>
If you have had this plugin installed since before version 2.5 but are running version 4.4 of WordPress, it is important that you leave the plugin installed. This is because all versions of the plugin before version 2.5 relied on a <code>data-sizes</code> attribute being present on an image in order to provide the responsive markup needed. If the plugin in this case is removed, then images in posts will be left with invalid markup. We are working to address this issue, and you can keep track of our progress here at https://github.com/ResponsiveImagesCG/wp-tevko-responsive-images/issues/178.
You can still use the plugin for advanced image compression support or as a simple way to include the picturefill script. The plugin will fall back to WordPress default functions if responsive image support is detected in your installation.
<ul>
<li>
Version 3.1.0 includes important changes that make this plugin compatible with WordPress version 4.4. Upgrading is highly recommended.
</li>
<li>
As of version 2.5.0, the plugin adds <code>srcset</code> and <code>sizes</code> attributes to images on the front end instead of adding them to the image markup saved in posts.
</li>
</ul>
Full documentation and contributor guidelines can be found on <a href="https://github.com/ResponsiveImagesCG/wp-tevko-responsive-images" rel="nofollow ugc">Github</a>

RICG Responsive Images

Bringing automatic default responsive images to WordPress.

Simages vs RICG Responsive Images