WordPress.com now provide a new feature to allows you to add sharing functionality from a range of services to the bottom of your blog posts, to share the posts on your blog with friends. The services are: Facebook, Twitter, Press This, Reddit, Digg, StumbleUpon, Email to a friend, and Print. You can control these new options in your dashboard by going to Settings > Sharing.
You can choose which of these services you want to display and in a range of different formats. If you've custom CSS upgrade you can go even further and choose your own icons and layout. Also added stats so you'll be able to see what content people are sharing and on what services.
Here's a quick video introduction: