"The proposed HTML "picture" element (see responsiveimages.org, but has not been accepted for HTML 5) is implemented as a display formatter for image fields. This module is a backport of Drupal 8 picture module. This module will deliver alternate image sources based on device capabilities to prevent wasted bandwidth and optimize display for both screen and print. "

"Set JPG quality per image style
This module allows you to specify a custom quality on different image styles you create. After installing and enabling this module, you will have the option to add a new effect to your image styles, enabling you to change the quality."

"Defines a field type Block reference which allows the block to be displayed as the content of the field." Alternately, if the bean module is enabled, blocks can be referenced using the entity reference module instead.

Provides token support for the upload directory name and file name.

This module provides site builders with a way of 'attaching' blocks within the content of any fieldable entity in Drupal. The same block is shown on any entity of that type/bundle.

"CER keeps reference fields in sync. If you have two entities that refer to each other using Entity Reference (or some other kind of reference field), it saves you the trouble of double-editing your entities in order to have them point at each other. If entity Alice references entity Bob, CER will make Bob reference Alice, and keep the references up-to-date when either entity is updated or deleted."

This module uses the FitVids.js library for fluid width video embeds (e.g. flash video in <iframe>s). You don't need it for pure HTML5 videos.
It supports YouTube, Vimeo, Blip.tv and Kickstarter by default, and can use it with other video providers like Dailymotion or Viddler.

Smart Trim implements a new field formatter for textfields (text, text_long, and text_with_summary, if you want to get technical) that improves upon the "Summary or Trimmed" formatter built into Drupal 7

This module is basically the Drupal 7 successor to CCK 3's Multigroup in Drupal 6. A similar module is the Inline Entity Form. Caution: ...

Let's you create, edit, and delete entity references from any form that includes an entity reference field.

This module might not work well with translation and workbench moderation.

Also see Field Collection