TwitThis is actually more of a blogging tool rather than a Twitter tool. It's a good utility for anyone who has a blog or Web site and is looking for more methods of promotion. It's also useful for people who like to share interesting things they run across online.
TwitThis provides HTML code you can add to your Web site. This code puts a button on your Web page or blog. If a reader wants to share your site via their Twitter account, they just need to click the button. That click will publish your URL to the user's Twitter feed, and voila -- your Web site gets instant promotion without you having to do a thing.
If you don't have a Web site you want to promote, you can still use TwitThis as a bookmarklet. A bookmarklet is a one-click tool that sits in your browser's toolbar. With the TwitThis bookmarklet, if you're surfing the Internet and see something you'd like to share on Twitter, just click, and the URL will post directly to your feed -- and you never even have to leave the page you're viewing. And TwitThis automatically shortens the URL, so you don't have to worry about it taking up your entire Twitter 140-character limit.