Jetpack for WordPress is a great plugin for any WordPress site, it's made by the people who create WordPress.
Recently it's got an update with some very notable features. Being updated to v2.0 it now has social sharing features. So every time you publish a new post it can share them automatically for you. It can share those items to Twitter, Facebook, LinkedIN, Tumblr and Yahoo!.
Other updates include Photon, Post by Email and Infinite Scroll.
Photon now enables users to use WordPress.com's servers to take care of their images. This will ease the load on your server and it's completely free.
Post by email now allows you to publish new posts by sending them to a secret email which you are given.
Infinite scroll isn't something you usually see in WordPress but with the feature becoming popular elsewhere around the web, especially in apps, Jetpack now enables sites to have Infinite Scroll. But your site has to be coded to use this feature.
There are a number of other notable features which are in Jetpack such as mobile view, email subscriptions, gallery lightbox and much more.
I use it on UltraLinx and highly recommend anyone else with a WordPress website to use it too.