Source: Send from Gmail (by Google) – Chrome Web Store

Once installed, the Send from Gmail Chrome Extension lets you quickly send a link to the current page with Gmail. Simply click on the button in the toolbar and a new email window will pop up allowing you to customize the email message before sending.

There are two options you can set by right-clicking on the toolbar icon and selecting options.

The first is to set your default Google Apps Domain. The second option is more interesting, allowing you to automatically set a prefix to the subject, such as Sharing:.