Smooth Scroll Up
Add a simple "Scroll to Top" button to the bottom right of every one of your posts. There are three styles to chose from, and everything is kept very lightweight. Typography and colors are inherited from your theme styles.
Front-end Editor
Edit your post without every having to go to the back-end. While browsing your site, this plugin will enable a small "Edit" button next to your post that will open a WYSIWYG editor inline, in the front-end of your post.
Easy Digital Downloads
Sell Digital goods on your WordPress site with the ability to set up multiple payment gateways, bundle together multiple files, and track order and download history.
WP To Twitter
Sync your WordPress site with the Twitter API and customize which posts should be automatically tweeted to your account. Control the content of your tweets, and change settings on a post to post basis with a built-in meta box in your post editor.
Better WP Security
Gives you a list of security tweaks that you can make for your website in a variety of different categories. Also performs regular database backups in case the worst happens and you need to restore everything.
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.