[superuser2]

Something like this already exists. Your browser can already identify itself with a certificate.

The interface is ugly and cumbersome for the non-technical (even in Firefox) and I don't think it's used very often, but I know that MIT issues certificates to students which identify them to all MIT's web applications, as well as third parties like the Apple and Dell student discount pages.

[carbonica]

Dartmouth does this as well, and I agree the interface sucks in browsers. Especially since most browsers will gladly show expired certificates for you to choose so you have to manually prune your list.