WordPress.com now offer a twitter button to bloggers.
“How it works: When one of your readers hits the Tweet Button, they will be shown a popup that includes a shortened link to your post. Readers can add in a quick message, and then hit “Tweet” to send the post to their Twitter feed as a tweet — all without leaving your blog.”
If you want to install the new Twitter button on your WordPress.com blog:
Go to Appearances, then Extras in your menu,
Click on the “Show a Twitter Tweet Button on my posts” button.