The God Login
Great post by Jeff Atwood on log in forms:
Logging in involves a simple form with two fields, a link, and two buttons. And yet, after reading all this, I’m sure you’ll agree that it’s deceptively complex. Your best course of action is not to build one at all, e.g. rely on authentication from an outside source whenever you can.
I’m not sure, if I agree with his conclusion here, but I agree it’s very complex.