There are many twitter plugins for WordPress.
Post twitter via WP, when you post a new article. It updates Twitter when you create a new blog post or add to your blogroll using Cli.gs. With a Cli.gs API key, creates a clig in your Cli.gs account with the name of your post as the title. I just installed this plugin but there is a little bugs with WP 2.9. Settings don’t display while clicking the toggle menu, so I removed javascript in wp-to-twitter-manager.php.
It displays your latest twitters with a twitter widget. There are other plugins like this, and you choose one of them that is right for you.
Twitter Tools include both read and post twitter functions. It is developed and supported by bit.ly. And I think this plugin is better than others.