Convert the following formats:
- @username -> http://twitter.com/username
- @username/listname -> http://twitter.com/username/listname
- #hashtag -> http://search.twitter.com/search?q=%23hashtag
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.
Te puedes descargar este plugin para utilizarlo en tu instalación autoalojada de WordPress.