I was thinking of writing a spec and implementation on a similar thing a few days ago:

Each site would publish a standardized file in a standardized location, and/or add a META tag to the file in each page. The file itself would simply contain the URL to the login page and the parameters that should be passed to it, and the way to pass those parameters.

Then, your browser/password manager could simply perform a request in the background, and log you in completely automatically. This solves all the hassle of finding the login form, locating the fields, auto-filling, typing some characters if the change wasn't detected, handling single-page apps, etc.

This could later be extended to signing up automatically. It would make having strong, unique passwords for each site the convenient alternative.