from day-one (Oct 2008), I designed the login form on http://costpad.com to be

  - embedded in every page so the user logs in whenever he/she wants
  - always shown so a single click starts the process
  - keyboard-friendly by using Tab to navigate to the input boxes
  - Javascript-less friendly, so to be able to log in without JS 
   and not having to "bypass" my beloved NoScript
  - big-styled (as the rest of the webapp) so to be tablet/finger friendly.

Yes, this implementation occupies quite a lot of space up there, but I think the trade-off is fine.

The popup/dropdown login form is certainly cool and space-saving, but I couldn't come up with an implementation that satisfies me...