Social Sharing by Danny
Add share links for sharing on Twitter, Google+ and Facebook to the bottom of your posts and pages in WordPress. Very lightweight, and you can optionally load in icons with CSS and pop-up windows with JS.
Wordfence
Scan your site for any malicious code or errors that might be occurring. You can use the plugin to monitor live traffic, block certain IPs from accessing your site and enable quick fixes for any problems that might arise.
Iframely Responsive Embeds
Take the oEmbed functionality of WordPress and make it automatically responsive. It uses the Iframely API so there is support for literally hundreds of link types, including YouTube, Vimeo, TED, and more.
System Snapshot Report
Get information about your WordPress install, with additional information about your server and browser, including detailed info about PHP configuration and WP Config info.
Google Doc Embedder
Embed a variety of filetypes (PDF, DOC, CSV, etc.) right in the browser using the Google Viewer API. Embed documents with a shortcode or an extra button in the WYSIWYG Editor.
Attachments
Associate posts with media attachments and metadata, then output this data in your theme files using an extensive API with several functions and a custom Loop.
Plugin Protector
Add a notice to any plugin in your WordPress install. That notice will be displayed to any person trying to update or delete the plugin. Good for installs with multiple users with admin access.
BWP Google XML Sitemaps
Get full control over your XML sitemaps, used by Google and other search engines to crawl and index your site. Support for multisite, multiple taxonomies and optional Google News add-on.
WP Caregiver
Change basic settings on the back-end and front-end such as removing the admin bar, remove header links, disable notifications and even get rid of the WordPress version link.
Theme Mentor
A perfect companion to Theme Check, find some common errors that plagues theme development, and easily find where in your theme file the problem is located so you can make it a quick fix. Check any theme installed on WordPress.
YOP Poll
Easily create polls on your WordPress posts and pages, and put them anywhere with a shortcode. The plugin can be as robust as you need it to be, and provides plenty of options for expanding the functionality and customizing your polls.
All in One Schema.org Rich Snippets
Use search rich microfomrats snippets from Schema.org to add a new dimension to search engine results. Customize the output of the rich snippets, and add to any post with a simple metabox.