Colllor - The favicon changes to match the chosen color
Github - The private label of Github adjusts its angle with an animation to fit your browser window.
GitHub - shows an infinity symbol when the number of notifications is over 800.
Gmail - Selecting an email in Gmail and then shift clicking on an email below it will also selects all the email between them.
/via Guy Malachi
Facebook - The icon for marriages is automatically based on the genders of those involved.
Facebook - When you type in the correct password with a slightly incorrect username, it will prompt you with the correct username and ask you to retype the password.