I use this in one of my sites:
Ingoal’s Twitter Updater
I just installed and tested Tweetly Updater. It automatically sends a Tweet whenever you publish a new post and includes the post title and a bit.ly link. Sweeeeet!
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin
I currently use Twitter Tools , it gives an option whether to tweet a particular post when publishing a new post. It also automatically shortens the URL using bit.ly. Its completely automated and doesn't need anything on your behalf. You can also write tweets directly from your WP Admin panel.
Just install the plugin and insert your twitter user and pass. That's all you have to do in the 'setting up' process.
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I don't remember which one I tried once. But It had a problem. When you went to save for publish date it tweeted the post before the actual date published.
Have any of you tried this with the one your using? Is that bug fixed?
There's lots of plugins for twitter. I'm using TweetMeme - Search and Retweet the Hottest Stories on Twitter