It's too bad that WordPress.com doesn't yet offer the ability to add plug-ins other than the ones they've decided to official support. If you are running WordPress on your own machine, this isn't a problem.
Sociable – Push cx
Social bookmarking sites allow websurfers to save, catalog, and share interesting pages they find online. The Sociable plugin appends links for your readers to use those sites to the end of each of your blog’s posts, increasing your potential audience.
The widgets plug-in for sidebars should support this.