WordPress.com has introduced a like and reblog feature. When logged in to WordPress.com and viewing a post a new link appears in the admin bar at the top of the page. If you liked the post you can click the “Like” link to show this. This shows the author how many readers liked the post.
You can reblog it to the readers of your own blog, by clicking on ‘Reblog this post” The new post will include a piece of the original text, and links back to the blog and specific post.
More info: http://en.blog.wordpress.com/2010/06/01/we-all-like-to-reblog/