Sugar Events Calendar Lite
Set up events in a custom post type, with details such as time, date and details of event. Then, use a shortcode to add an AJAX-enabled, simplistic calendar to posts or pages.
Advanced WP Columns
Create responsive columns in WordPress posts and pages using a GUI in the WYSIWYG editor with some very simple markup. Settings to customize output.
Query Monitor
Get a running list of MySQL queries on each page using a link in your admin bar. Groups queries together by different parameters and troubleshoot performance issues and errors.
Stream
Get a list of any updates made on your WordPress sites, including login attempts, new posts and pages, plugin activity and more. Filter activity based on action type.
Add IDs to Header Tags
Every header in WordPress posts will have a unique ID tag added to it, so that it can be easily linked to using a hashtag URL format. There will also be inline links for easy bookmarking.
Instagrate
Easily pull a stream of photos from your instagram account that you can display on your WordPress site in the form of galleries or individual posts with support hashtags and comments.
TinyMCE Spellcheck
Spellcheck used to be bundled with After the Deadline, before it became part of Jetpack. This plugin is a fork of that functionality to add it back to your WYSIWYG editor and can be customized for each user.
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.