knuten

This plugin let you add a shortcode with two different image sources one too a normal sized image and one for retina. The plugin will then load the right image depending on if it’s a retina display or not. 
For more info and how to use it.
<h3>A brief Markdown Example</h3>
<pre><code>[retina normal="path-to-normal-image" retina="path-to-retina-image" width="image-width" height="image-height" /]
</code></pre>

Retina for WP

This plugin let you add a shortcode with two different image sources one too a normal sized image and one for retina.

ArrayHQ

kyleunzicker

Okay Portfolio Items
If your theme supports portfolio items, the Toolkit adds a Portfolio custom post type. Once enabled, you will see a Portfolio Items menu item, which you can use to create your portfolio.
Okay Custom Image Galleries
If your theme supports custom galleries, the Toolkit adds custom image gallery support. Once enabled, you can create custom, editable and rearrangable image galleries using the new 3.5 Media Manager.
Okay Widgets
The Toolkit adds Twitter, Flickr, Dribbble and Retina social media icons to your theme.

Okay Toolkit

The Okay Toolkit extends the functionality of Okay Themes. It provides a portfolio custom post type, slider gallery support and social media widgets.

Retina for WP vs Okay Toolkit