When GMail came out, invites were the big thing just to see Google's new web mail. Now, it seems that everyone can't get enough of GMail extensions. The one problem with GMail is that is doesn't look like it's that secure (http, not https).
GMail S/MIME provides a secure method of sending your email without worrying about snoopers. Install the extension and restart Firefox.
You won't see the extension doing anything for you until you go into your GMail Web Mail. If you are using a email client like Outlook or Thunderbird, this will work for GMail as well.
This is not your basic extension that you can just install and go. You need a personal certificate from one of the security experts like Thawte or Verisign. The way to receive your certificate is already well documented and takes a little more time before you can begin sending secure emails. Read the document on how to create an S/MIME Certificate.
If you use the web mail portion of GMail and your certificate is properly installed in Firefox, when you compose an email in GMail, you'll notice a lock in the top right corner of the body area where you compose your main message. You'll notice a lock is enabled. Compose your email and send it on its way and you should feel secure (sorry, bad pun) that your email won't be viewed by anyone else but you and the recipient.
A couple of downsides to the extension are that file attachments aren't supported in this version and the padlock doesn't appear in Firefox 1.5 (stick with Firefox 2.0).
Are they going to provide support for file attachments in a later version? I sure hope so. This extension is the closest thing I've seen for sending secure GMail.
GMail S/MIME is a great extension for providing a piece of mind to make sure your emails aren't read by prying individuals.
GMail S/MIME receives a 4 out of 5 rating.