Contextual Related Posts
Add related posts to the bottom of your pages, which are based on post title and post content rather then just categories and tags. You can chose which post types to add to, customize the output, or implement the plugin manually.
Regenerate Thumbnails
Generate new thumbnails when new image sizes are added to your WordPress theme. You can regenerate all of your thumbnails at once, or chose just a few, or even just one image using the Media Library.
Duplicate Post
Clone your WordPress posts from a few different locations in your admin panel, and customize the outputted title of the post. Options to edit a post right away, or save it as a draft and come back to it later.
WP-CLI
Add command line tools to WordPress. Update your WP site, install one from scratch and manage databases without leaving the terminal. If you find yourself completing dev. tasks over and over, this is probably the right plugin for you.
Enable Media Replace
Swap out images and assets you have already uploaded without having to delete images and replace them manually. Gives you access to a new option in your Media Library entitled "Replace Media."
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.