Social bookmarking is a great way to improve the traffic to your website, because put quite simply it is free advertising. This is not too hard for most people, just open up the page in an editor and paste in a social bookmark code. However, on WordPress.com, the system is closed in a way that makes it difficult.
Enter, AddToAny. AddToAny has a bookmarklet (a script that is activated when you click on a bookmark) that can be put in any desktop web browser. Just start a new post, then click HTML and click at either the top or the bottom of it (pick where you want the button) and then just click the bookmarklet in your bookmarks bar. You now have a social bookmarking widget that your visitors can use to tell their friends about your site.
Check it out at: http://www.addtoany.com/buttons/for/wordpress_com
Unfortunately, I do not know of a way to automatically put this into every post. Let me know in the comments if you know of a way.
UPDATE: You can use the widgets page in your WordPress control panel to put the share button in your sidebar, but the advantage of a button like the one below is that is shares the permalink, not the homepage like one in the sidebar would do when reading a post on the homepage.