deviantArt - Visual prompt to encourage users to fill in more details about their profile when registering.
/via Benje
Twitter - Tracks accounts that are viewed by unregistered and logged out users. Then prompts them to sign up and follow those accounts automatically.
/via Jon Wheatly
Wunderlist - The desktop version has one form for both signup and login.
/via Denise Pires
Lego - The lego character covers his eyes when a user types in their password.
/via acmeideas
Sign in button expands and folds out once clicked.
Try it for yourself here
/via Matthew Farag
Quora - After entering your email adress it loads your avatar to the right of the form.
Twitter sign up password form