Usernoise
Creates a simple feedback form, embedded in a responsive, mobile-friendly modal box. On each page, a Feedback button will direct users to a customizable form with several different categories.
SyntaxHighlighter Evolved
Highlight code samples right within your post editor. Simple to set up, uses shortcodes to define code language, and lots of customizable options.
Picturefill.WP
Picturefill.WP provides an easy way to add responsive images to your WordPress site, using the Picturefill polyfill developed by Scott Jehl and automatic image sizing.
W3 Total Cache
W3 Total Cache is a "performance framework" that allows you to enable and tweak Page, Database and Object caches, utilize Browser compression, integrate with your CDN and plenty more.
Jetpack
The swiss army knife of plugins, the Jetpack plugin comes with a few dozen features to enhance your WordPress site, from carousels and comments to a full JSON REST API.
Hotfix
Used to serve non-critical updates to those that use it when WordPress is in between development cycles. Automatically detects your WordPress version.
Breadcrumb NavXT
Add breadcrumb navigation to your WordPress install. Customize the look and display of your breadcrumbs in your admin panel and then add a single function to your template files.
Members
Access to a role and user management GUI and several tools to make the process easier. Create new roles and assign custom capabilities from within your admin panel.
WordPress SEO by Yoast
Access tools that increase your presence in search engines. Edit titles, generate XML sitemaps and edit .htaccess and robots.txt files.
Gravity Forms
Create new forms with a drag and drop interface, add them to your post editor with a shortcode or template files, set up admin notifications, and customize every step of the way.
WP Migrate DB Pro
WP Migrate DB sync up local and production MySQL databases with one-click. It provides automatic search and replace, transfers over HTTP/HTTPS and easy push, pulls functionality using built-in API key.
Advanced Custom Fields
Make custom fields in WordPress easy and quick. You'll get set-up with a GUI for adding new fields, and functions to incorporate them into your themes.