WordPress.org

1.8.3: 2011-07-05: Small update to fix compatibility with WordPress 3.2.

1.8.2: 2011-02-21: This is a nonessential update to increase my revenue. ;)
* Changed AdSense option to Amazon Associates, since Google banned me.
* Changed donation links to differentiate between > $11.90 and < $11.90.
* Changed the default URL service from local to is.gd, contrary to popular
  demand for bit.ly, because bit.ly now requires an API key.
* Made Step 4 red under Automatic Tweeting to cut down on confusion.
* Updated options to indicate Bit.ly credentials are now mandatory.

1.8.1: 2010-11-06: Bugfix release for Twitter "Snowflake" status IDs.
* Critical bugfix: Tweet This gets the status ID from id_str instead of id to
  be compatible with the new Twitter "Snowflake" IDs rolled out Nov. 4, 2010.
  Before, an ID such as 549919521771520 would be stored as 5.49919521772E+14.
* Critical bugfix: I have tested the new usage of add_meta_box and have found
  it only works in WP 3.0 or newer. The Tweet This box would not appear at all
  on old versions of WordPress, so I've added a version check to return to the
  Tweet This 1.7.7 behavior on WordPress 2.7 - 2.9.2.
* Added "Insert Tweet This block before/after content" checkboxes to the
  Advanced Options, so you can insert links before posts, after posts, or both.
* Tweet This can now be hidden on RSS feeds in the advanced options.
* Posts can now be retweeted by checking "Send to Twitter again" when editing.
* The date of auto tweets are now saved in the custom field tt_tweeted_date.
* Reloading the Write Tweet page now forces all scheduled tweets to fire, even
  if it has been fewer than ten minutes since the last check.
* On the Write Tweet page, published scheduled tweets now display a link to the
  Twitter API XML file in the Status column. If the tweet failed, the HTTP
  error code is displayed when hovering over "fail" in the same column.
* If your blog uses www, it is now stripped from local URL, saving 4 chars.
* Bugfix: "Use full permalinks unless Tweet/Plurk Text exceeds 140 characters"
  caused malformed URLs on automatic tweets, so it no longer applies there.
* Bugfix: Single and double quotes manually entered into the Tweet This box on
  the Write Tweet page would be escaped with backslashes in previous versions.
  Now, the PHP function stripslashes is run on the entire tweet.

1.8: 2010-10-02: Major release with many new features and improvements.
* Added Bebo, Diigo, FriendFeed, Gmail, Google Buzz, LinkedIn, Mixx, Slashdot,
  Squidoo, and Technorati to the Extended Services, bringing the total to 20.
* Added "Public Display Order" field to the Extended Services section, so you
  can change the order in which the services are displayed.
* Updated the auto-tweet options on the Write Post screen to use add_meta_box()
  so you can move the box around. Changed the default position from "advanced"
  to "side" and changed the text input to a textarea.
* You can now use [AUTHOR], [CATEGORY], [DATE], [TIME], [EXCERPT], and
  [BLOG_TITLE] shortcodes in Twitter and Extended Services text fields.
* Scheduled tweets can now be deleted from the Write Tweet page.
* The icons directory has been organized into directories by language and
  service. Your options and icon display will be updated automatically.
  However, if you want the icons to continue to be accessible at the old URLs
  (i.e. for Feedburner email newsletter readers), please merge the new icons
  directory with the old directory instead of overwriting it.
* Added contrast and brightened some icons, and improved transparency.
* Added [tweet_this] shortcode which you can use in any post or page to display
  the Tweet This div including all links. You will have to add a custom field
  titled tweet_this_hide with the value of "true" to stop duplicate insertion.
* Added three buttons to the save section of the options page: "Reset All
  Options Except Keys," "Flush URL Cache," and "Completely Uninstall Tweet
  This," which deletes all plugin options, custom fields, and database tables.
* Tweet This links now have the target="_blank" and rel="nofollow" tags added
  by default. This can be turned off in the advanced options.
* The "Don't shorten URLs under 30 characters" advanced option is now "Use full
  permalinks unless Tweet/Plurk Text exceeds 140 characters" and it's behavior
  has changed accordingly. This also applies to automatic tweets.
* Replaced tt-twitter2.png with the new official Twitter favicon.
* Added a JavaScript clock to the Write Tweet page based on the server's time.
* Added "button-primary" class to the save and import buttons in the options
  and the tweet button on the Write Tweet page, so the buttons are blue now.
* Increased font size on /icons/tt-su-big2.png from 7 to 9 and moved arrow up.
* Increased the add_action priority on tweet_this_css, tt_shortlink_wp_head,
  and tt_html_comment to 9 so they have priority over other plugins.
* Updated the icon selection and extended services section in the options to
  display properly at 1024x768 resolution on WordPress 3.0.
* Changed default CSS to include a 2-pixel margin on the left side of icons.
* Added @package and @since PHPdoc tags to Tweet This functions and classes.
* The number of options modified and the number of cached URLs flushed is now
  displayed after saving the options.
* You can now rename the /tweet-this/ directory and tweet-this.php file.
* Bugfix: Selecting "Share Links" for Twitter did not work at all in 1.7.7.
* Bugfix: get_tweet_this_url() now wraps URLs in urlencode() for Plurk to
  prevent http:// from becoming http:. Plurk has been completely unusable in
  all previous Tweet This versions for some time.
* Bugfix: Fixed Yahoo Buzz links which did not work in all previous versions.
* Bugfix: URLs containing a subdirectory with an underscore followed by another
  subdirectory would be shortened incorrectly on the Write Tweet page. Fixed.
* Bugfix: Each instance of the Twitter "Editable text box" now uses the post ID
  in HTML element IDs and JavaScript functions, to prevent invalid HTML and
  allow multiple text boxes to be displayed on the same page without conflict.
* Bugfix: get_tt_shortlink($post_id) is no longer affected by the option "Use
  'www.' instead of 'http://' in shortened URLs." It always uses http:// now.
* Bugfix: In 1.7.4 - 1.7.7, I accidently included the line
  "remove_action('wp_head', 'wp_shortlink_wp_head', 10, 0);"
  which removed the canonical link and shortlink from wp_head on all posts.

Older versions: http://richardxthripp.thripp.com/tweet-this-version-history/