Penango works in Gmail and Zimbra. We are adding more platforms soon. In this screenshot, Penango processes signed and encrypted messages in Gmail.
As a web browser extension, Penango can extend the native capabilities of the web browser. In this case, Penango supports drag-and-drop of attachments in Internet Explorer, with all of the fancy Windows Vista drag-and-drop handlers. Extra care is taken with attachments so that they never have to be uploaded to the server in unencrypted form.
Here is a message being composed to Russ Housley. It is right in Gmail, with no additional usernames or passwords to remember, and no additional websites to visit. It tells users all they need to know: who sent the message, and who else will be able to read it.
This is the options dialog box in Firefox. Penango supports some advanced features like encrypted drafts. With encrypted drafts, nobody else can read your secret thoughts but you--not even your receipients! The recipients will be added for decryption when you actually send your message.
In addition to Gmail, Penango works in Zimbra Collaboration Suite and Zimbra Desktop. 66 million users can't be wrong.
Here is the compose window in Zimbra. Penango adds "Add My Info (Sign)", "Encrypt", and "Encrypt Draft" buttons. Penango also supports multiple languages: this example is Spanish.