Jetpack by WordPress.com

Jetpack by WordPress.com is a plugin for WordPress that allows self-hosted WordPress sites to incorporate WordPress.com features.

Features

 * Publicize - post updates to social networks when you publish in WordPress. Currently supports Facebook, Twitter, Tumblr, Yahoo! and LinkedIn.
 * WordPress.com Stats - very simple and easy stats, gathered by WordPress.com and shown in the widget and dashboard views.
 * Notifications - Unified notifications between any WordPress site connected to your account. Mostly used to see comments from all the sites.
 * Jetpack Comments - Comment form with WordPress.com, Twitter and Facebook login options (in addition to the name and email fields for non-logged in visitors).
 * Subscriptions - Allows visitors to subscribe to new posts or comment replies through email, sent from WordPress.com.
 * Carousel - Full screen lightbox for viewing images, includes commenting and EXIF info.
 * Post by Email - Self-explanatory; never used.
 * Likes - A "Like" button for people with WordPress.com accounts. Another useless ego button.
 * Sharing - Sharing buttons for various social networks. Ability to add own icons and URLs.
 * Spelling and Grammar - Uses After the Deadline to check your posts. Useful and recommended.
 * VaultPress - subscription-based backup and security solution.
 * Gravatar Hovercards - Popups for Gravatar profiles; useless.
 * Contact Form - Simple way to insert a contact form into any post or page. Collects submissions in the Feedbacks section of the site.
 * WP.me Shortlinks - URL shortener; not recommended, as shorteners tend to create long term issues.
 * Tiled Galleries - Makes pretty tiled mosaics for photo galleries. Note: this will serve your photos from WordPress.com's CDN, even if you are not using the Photon service (see below). Makes it kinda creepy.
 * Custom CSS - Very cool CSS override plugin. Can also parse processed CSS: LESS and Sass.
 * Shortcode Embeds - Very useful shortcodes for various popular websites.
 * Mobile Theme - A mobile theme alternative. Not recommended; you should have a responsive theme that adjusts to different devices.
 * Beautiful Math - Allows for LaTeX embedding. Useful if that is your thing.
 * Extra Sidebar Widgets - A widgets package, more stuff to go in your sidebar.
 * Infinite Scroll - Loads the next page of posts when you get to the bottom of the screen. Breaks on some themes, kinda useless.
 * Photon - Sideload images into WordPress.com's CDN. Useful if you don't have access to a CDN otherwise.
 * JSON API - An API that allows apps to access your site in new and interesting ways.
 * Mobile Push Notifications - Push notifications for iOS devices. If you are an Apple customer, this may be useful.
 * Enhanced Distribution - "Jetpack will automatically take the great published content from your blog or website and share it instantly with third party services like search engines, increasing your reach and traffic." No idea how that works!

2.2.5

 * Enhancement: Stats: Counting of registered users' views can now be enabled for specific roles
 * Bug Fix: Security tightening for metadata support in the REST API
 * Bug Fix: Update the method for checking Twitter Timeline widget_id and update coding standards
 * Bug Fix: Custom CSS: Allow the content width setting to be larger than the theme's content width
 * Bug Fix: Custom CSS: Fix possible missing argument warning.

2.2.4

 * Bug Fix: JSON API compat file include was not assigning a variable correctly, thus throwing errors. This has been resolved.

2.2.3

 * Enhancement: Comments - Add the reply-title H3 to the comment form so that themes or user CSS can style it
 * Enhancement: Custom CSS - Support for the CSS @viewport
 * Enhancement: JSON API - Support for i_like, is_following, and is_reblogged
 * Enhancement: JSON API: Custom Post Type Support
 * Enhancement: JSON API: Meta Data Support
 * Enhancement: JSON API: Bundled Support for bbPress
 * Enhancement: JSON API: Additions of following, reblog, and like status for post endpoints.
 * Enhancement: Shortcodes - Add Bandcamp shortcode
 * Enhancement: Tiled Galleries - Add code to get blog_id
 * Bug Fix: Carousel - Support relative image paths incase a plugin is filtering attachment URLs to be relative instead of absolute
 * Bug Fix: Carousel - Add likes widget to images / Respect comment settings for name/email
 * Bug Fix: Carousel - Make name and email optional if the setting in the admin area says they are
 * Bug Fix: Contact Form - Bug fixes, including a fix for WP-CLI
 * Bug Fix: Contact Form - Remove deprecated .live calls, delegate lazily to jQuery(document) since it's all in an iframe modal
 * Bug Fix: Contact Form - RTL styles
 * Bug Fix: Contact Form - Better handle MP6 icons
 * Bug Fix: Custom CSS - array_shift took a variable by reference, so avoid passing it the result of a function
 * Bug Fix: Custom CSS - Allow case-insensitive CSS properties (ref)
 * Bug Fix: Infinite Scroll - Maintain main query's post__not_in values when querying posts for IS
 * Bug Fix: Infinite Scroll - Ensure that IS's pre_get_posts method isn't applied in the admin. Also fixes an incorrect use of add_filter where add_action was meant. Fixes #1696-plugins
 * Bug Fix: Infinite Scroll - CSS update - IS footer was too large in Firefox
 * Bug Fix: Infinite Scroll - Add bundled support for Twenty Thirteen default theme
 * Bug Fix: Infinite Scroll - Include posts table's prefix when modifying the SQL WordPress generates to retrieve posts for Infinite Scroll
 * Bug Fix: JSON API - Use wp_set_comment_status to change the comment status, to make sure actions are run where needed
 * Bug Fix: Likes - Update style and logic for matching id's
 * Bug Fix: Mobile Theme - Ensure that minileven_actual_current_theme is child-theme compatible + other updates
 * Bug Fix: Mobile Theme - Update method for finding currently active theme.
 * Bug Fix: Notifications - Remove the postmessage.js enqueue since this feature solely supports native postMessage
 * Bug Fix: Notifications - Clean up script enqueues and use core versions of underscore and backbone on wpcom as fallbacks
 * Bug Fix: Notifications - Enqueue v2 scripts and style
 * Bug Fix: Notifications - Prefix module-specific scripts and style to prevent collision
 * Bug Fix: Notifications - Include lang and dir attributes on #wpnt-notes-panel so the notifications iframe can use these to display correctly
 * Bug Fix: Open Graph: Use the profile OG type instead of author. Add tags for first/last names
 * Bug Fix: Publicize - Remove the Yahoo! service because they stopped supporting that API entirely
 * Bug Fix: Publicize - fix fatal errors caused by using a method on a non-object. Props @ipstenu
 * Bug Fix: Sharing - Adding 2x graphics for Pocket sharing service
 * Bug Fix: Sharing - Bug fixes, and a new filter
 * Bug Fix: Shortcodes - Audio: make sure that the Jetpack audion shortcode does not override the 3.6 core audio shortcode. Also, we need to filter the old Jetpack-style shortcode to properly set the params for the Core audio shortcode.
 * Bug Fix: Shortcodes - Audio: Re-enable the flash player
 * Bug Fix: Shortcodes - Slideshow: RTL styling update
 * Bug Fix: Tiled Galleries - Fix IE8 display bug where it doesn't honor inline CSS for width on images
 * Bug Fix: Tiled Galleries - Remove depreacted hover call, use mouseenter mouseleave instead
 * Enhancement: Twitter Timeline Widget: New JavaScript based widget. Old one will discontinue May 7th.

2.2.2

 * Enhancement: Mobile Theme: Add controls for custom CSS.
 * Enhancement: Sharing: Add Pocket to the available services.
 * Bug Fix: Custom CSS: Update the method for generating content width setting.
 * Bug Fix: JSON API: Security updates.
 * Bug Fix: Likes: Add settings for email notifications and misc style updates.
 * Bug Fix: Notifications: Add the post types to sync after init.
 * Bug Fix: Publicize: RTL styling.
 * Bug Fix: Shortcodes: security fixes and function prefixing.
 * Bug Fix: Widgets: Update wording on the Top Posts widget for clarity.
 * Bug Fix: Jetpack Post Images security fixes.