Nolongeractive

The plugin picks out twitter usernames and hashtags and automatically converts them to working links to Twitter profiles and search pages. You will no longer have to manually create these links, it will happen automatically for all page, post and comment content.

Automatic Twitter Links

This plugin automatically converts Twitter usernames and hashtags to Twitter profile- and searchlinks in pages, posts and comments.

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.

Automatic Twitter Links vs Twitter Highlight