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.

zackproser

The Twitter Hashtag Blaster enables your readers to send your content into Twitter conversations surrounding your target hashtag.
Do you run a humor website? Set your hashtag to #lol and start enjoying increased traffic from Twitter. Is your content about #football? #programming? #fashion? #politics? You can target any hashtag you like.
Once installed and activated, go to Settings -> Twitter Hashtag Blaster.
Here you can enter your target hashtag and choose whether the button should be small or large.

Twitter Hashtag Blaster

Add Twitter hashtag buttons at the end of every post, for any hashtag you want. Sends shortlinks to your articles into Twitter!

Twitter Highlight vs Twitter Hashtag Blaster