Eduardo Reveles

Convert the following formats:
<ul>
<li>@username -> http://twitter.com/username</li>
<li>@username/listname -> http://twitter.com/username/listname</li>
<li>#hashtag -> http://search.twitter.com/search?q=%23hashtag</li>
</ul>
It comes with some options to configure, like use nofollow attribute, open on a new page and choose where you want the links to be replaced.

Twitter Highlight

Convert twitter usernames, hashtags and lists in pages, posts or comments to a twitter link.

Darkseal

IMPORTANT: This plugin requires Jetpack’s Publicize module: read below for additional info.
As the name says, this plugin will automatically append hashtags to any content sent by Jetpack’s Publicize module. This means that the plugin will work only if the Jetpack plugin is already installed on your WordPress platform with the Publicize module set to active. Don’t worry, tho: if any of these requirements are missing this plugin won’t do anything, without breaking your blog or compromising its performance in any way.
Plugin Features
<ul>
<li>Hashtags will be created using the post <code>tags</code>, (optionally) trimming the space chars, until the maximum character length is reached (if any).</li>
<li>You’ll still be able to manually add custom hashtags, pre-pending or appending them to the auto-generated ones just like you always did.</li>
<li>You can reorder the hashtags as you like, regardless of how they were added.</li>
<li>Dupe check included, ensuring that the same hashtag won’t be included multiple times – unless you manually do that.</li>
<li>Automatically added hashtags won’t break the 118-character limits of the social message text to keep the message length under the standard tweet + link limit.</li>
<li>The above character-length threshold can be changed and/or set to infinite with ease.</li>
</ul>
Useful Links
<ul>
<li><a href="https://plugins.svn.wordpress.org/publicize-with-hashtags/" rel="nofollow ugc">Plugin SVN Repository</a> hosted by WordPress.org (in real-time sync with the GitHub repo).</li>
<li><a href="https://github.com/Darkseal/publicize-with-hashtags" rel="nofollow ugc">Plugin GitHub Repository</a> (in real-time sync with the official SVN repo).</li>
<li><a href="http://darkseal.github.io/publicize-with-hashtags/" rel="nofollow ugc">Plugin GitHub Page</a>.</li>
<li><a href="http://www.ryadel.com/works/publicize-with-hashtags/" rel="nofollow ugc">Plugin official page</a> at <a href="http://www.ryadel.com/" rel="nofollow ugc">Ryadel.com</a>.</li>
<li><a href="http://www.ryadel.com/en/add-hashtags-to-jetpack-publicize-shared-posts/" rel="nofollow ugc">Installation & Usage Guide</a> at <a href="http://www.ryadel.com/" rel="nofollow ugc">Ryadel.com</a>.</li>
</ul>
<h3>Authors</h3>
<ul>
<li>Darkseal of <a href="http://www.ryadel.com/" rel="nofollow ugc">Ryadel.com</a>.</li>
</ul>

Publicize With Hashtags

Automatically append hashtags to content sent by Jetpack Publicize module. Hashtags will be created using post tags. Dupe and length check included.

Twitter Highlight vs Publicize With Hashtags