The HashTag plugin is yet another twitter plugin. It will Tweet the entry title and a link using the account in the plugin settings, it can also add hashtags to your tweets. Hashtags are Twitter's version of tags, they provide a way of adding structure to your tweets and allowing people to filter tweets against their interests.
The latest version of HashTag can be downloaded from http://www.composing-stick.net/projects/hashtag/. NOTE this version is for MT4.x. There will be a separate version released for MT5.x as there are some changes to the edit entry screen which means that the tweet options are not displayed correctly.
The source code can be found at http://github.com/TheComposingStick/HashTag