I've created a plugin called Tweet This, which lets your readers link to your posts in a Twitter tweet easily. It even pre-trims the URLs, so they have plenty of space to write without going to TinyURL or another redirection service manually.
Here's the long description:
Adds a Twitter icon to every post and page, with a nice rollover effect. URL trimming by th8.us. If logged in, the reader will find the Twitter.com tweet box pre-filled with “www.*****.th8.us “. No configuration options. The icon appears at the top-right of each post.
Normally, posting a link to Twitter eats up your 140 characters. Though they shorten URLs with TinyURL, it doesn’t happen till after you post your tweet, so you don’t have many characters to work with.
While you could go to TinyURL.com, copy and paste the blog post’s permalink, shorten the URL, copy that new URL, go to Twitter, and paste it into the box, this plugin merges all that into one step, using the Twitter API (a.k.a. an HTTP GET request) and th8.us, an external URL trimming service I created. I didn’t like how long TinyURL’s URLs are, so I made th8.us. Its URLs are 16 characters instead of TinyURL’s 25, giving you more space to write about the blog post. I use the www. prefix instead of http://, which saves three characters. Twitter picks up on it fine.
This plugin fetches a shortened URL from th8.us for each of your blog posts’ permalinks server-side, then displays a link to Twitter for each post, with a cute icon.
Released under Version 3 of the GNU General Public License: