fiskeben

Adds a link to each post that points the user to Twitter.com. Before the redirect, the URL for the actual post is sent to Tinyurl.com and the tinyurl-ified URL is used as an argument to Twitter.com thus putting it in the status field when the page loads.

FishyTweet

Adds a "Tweet this" link to posts, shortens URLs with tr.im or TinyURL and sends the user to Twitter.com.

marco_b

This plugin updates your twitter status with the latest post that you’ve made. It also has the option of including a link back to your post so people can easily click on it to view the post. 
The link provided in the tweet can be shorted via a service of your choice: tinyurl.com, is.gd, bit.ly or snurl.com
If you wan to stay up with me, you can <a href="http://twitter.com/marcobischoff" rel="nofollow ugc">follow me</a>. 
Enjoy!
<h3>Features</h3>
With this version it is possible to update more than one twitter account.
Still missing is the following: 
ow.ly as shortener service 
React on posts which are written for the future

Twittrup

Updates Twitter when you create a new blog post utilizing an shortener service of your choice.

FishyTweet vs Twittrup