If your tweets to your Twitter followers often include excerpts from Web pages, you'll welcome CloudBerry TweetIE. Given the simplicity of Twitter, you'd expect this Internet Explorer add-in to be simple to use as well--and you'd be right. Install it, and then tell Internet Explorer it as a toolbar. It displays as a small button. Click it to log in to Twitter. Once you've done that, when you find text on a Web page you'd like to send via Twitter, copy the text to the Clipboard, and it gets pasted into CloudBerry TweetIE, as does the URL. You can then edit the text, and send it as a tweet. It's that simple.
Note that there is also a version available for Firefox, but when I tried to install it, it refused to install. The Firefox add-in is also listed as an experimental add-in, so at this point I can't recommend trying the Firefox version. At some point, though, the problems might be fixed, and it would then be worth a try.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.