Twitterify for WordPress automatically converts hashtags (like #wordpress) to tag links, @links to author links (like @metinsaylan), and auto links URIs (like http://wpassist.me ) to links. Some benefits of Twitterify are:

Linking will take less time, you will get more productive.
Tag links will be fine even if you change tag permalinks later on.
All links in the content will automatically have no-follow attributes.

Installation

Go to Plugins > Add New
Search for twitterify
Install & Activate