Actually, while it's very rarely used, you can give your browser a certificate for you (such as a Thawte personal certificate) and it will do more than sign mail, it will authenticate *you* in SSL conversations with web sites -- as well as their own certificate authenticating them. So this could be used as a login/auth mechanism but nobody does it. If you look in your firefox preferences under "advanced/encryption" you will even see where you can set what to do if a web site asks for your certificate.