That is pretty much the epitome of "hard task". Sites differ, associating the right site with the right password is difficult, and the security risks (having scripts accidentally send the wrong creds to the wrong site(s)) mean you'd really want to get this right.
In conjunction with some other thoughts of mine, though, this suggests a space for a more API-driven web design generally. One in which the authentication mode is clearly expressed. HTML had this integrated at one point with htaccess, but it was a horribly simplistic model.
