The three factors are:
- something you know (password)
- something you are (biometrics)
- something you have
You want two factors and one cannot be "something you have." So you're looking at passwords and biometrics. I suppose a fingerprint scanner is out of the question, for the same reason as "something you have." Iris scanner, too, I suppose.
Typing pattern recognition? There are many products that can be used to implement this as your second factor.
I'm unclear why you're ruling out "something you have". Consider <a href="http://www.softek.co.uk/prod/ak/authentication_applications.asp">DeepNet Security</a>; the something you have could me a cell phone.
I'm unclear why you're focusing on" two factor authentication," either. Many people look at the problem of web application insecurity as a client authentication problem: "Can I trust that this person is who they say they are?" Then increase their confidence with two-factor authentication. Actually, security breeches are often better prevented with two way authentication (mutual authentication). This is typically done business to business ("Can I trust you?" "Yes, but can I trust you?"). There is no reason a business shouldn't recognize that it has an obligation to its customers. "Can I trust that the customer is actually connecting to my web application, not some man in the middle or impostor?" Extended validation of certificates (qualifying for the green bar in the URL box) is a partial measure. However, it relies upon the customer to enforce authentication, and that is not a reliable approach.
Two way authentication can be implemented using SSL <a href="http://www.entrust.net/certificate-ssl.htm">Entrust</a> <a href="http://developers.sun.com/appserver/reference/techart/mutual_auth.html">Mutual Authentication for Web Services: A Live Example</a>. At a minimum, be prepared to explain why you have chosen to <i>not</i> do mutual authentication.
Or if you want two way, two factor authentication in a more exotic way, see DeepNet or products of its ilk. Just as an example, not an endorsement.
Oh, right. <a href="http://blogs.msdn.com/danielmcpherson/archive/2006/02/16/533605.aspx">Sharepoint and multiple login requests</a> should be mentioned.