Jetpack is a WordPress plugin designed to bring WordPress.com power to your self-hosted WordPress sites. This supercharged plugin aims to provide blog site powered by wordpress, to enjoy the same functionality just like you are hosted in wordpress.com. Jetpack is developed by Automattic, the company who made WordPress. If you remember in WordPress 3.1 they also added Admin Bar which is also a feature of WordPress.com. Here is the list of all included features on Jetpack.
Simple, concise stats with no additional load on your server. Previously provided by WordPress.com Stats.
The WP.me URL shortener.
Hovercard popups for your commenters via Gravatar.
Easily embedded media from popular sites like YouTube, Digg, and Vimeo.
For the Math geeks, a simple way to include beautiful mathematical expressions on your site.
A widget for displaying recent tweets. Previously provided by Wickett Twitter Widget.
Your readers can easily share your posts via email or their favorite social networks. Previously provided by the Sharedaddy WordPress plugin.
Your writing will improve thanks to After the Deadline, an artificial intelligence based spell, style, and grammar checker. Previously provided by the After the Deadline WordPress plugin.