Meteor Slides
Add slideshows to your site using the jQuery cycle plugin. Pick from several different transitions, and organize slides into multiple slideshows. Add them to your posts or pages with a shortcode, function or widget.
Debug Bar
Places a button in your admin bar that gives you troubleshooting information for each page that you are on, including PHP Errors, SQL queries, HTTP Requests and a full server rundown. Develop WordPress sites faster and more efficiently, with all of your debugging info in one place.
Quick Page/Post Redirect
Add redirects to your site, either on a post by post basis using a metabox provided at the bottom of your post editor, or using the Quick 301 Redirects feature for any page or directory. Global override options are also available and customizable.
Relevanssi
Improves search automatically by building a more comprehensive index then WordPress out of the box. Options to customize what gets included in the index, caching options, and fine-tuned control, and works with existing search forms.
Editorial Calendar
Manage your posts using a calendar interface. Easily drag posts from one day to another, schedule your drafts and view all of your upcoming and past posts in a full page calendar layout.
Password Protected
Hide your entire WordPress site behind password protection. Any users visiting your site will be greeted by a login page which prompts them for a password that you specify.
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.