I have build a widget called CopyToClipboard. It can be found in GitHub at https://github.com/Luch/CopyToClipboard and hopefully soon in the App Store. The widget is using jQuery and ZeroClipboard libraries. Enjoy!
Copy to clipboard can potentially be dangerous, so most browsers don't really support it out-of-the-box.
The most common solution is to use a small flash plugin for this, like this one suggest in the SO answers: https://github.com/mojombo/clippy
We don't have a widget that implements this yet.
In the newest browsers (except Safari) there is now a HTML5-solution possible: http://zenorocha.github.io/clipboard.js/