Mixpanel - When changing your password, the field for the current password has an old style key, and the new password has a modern key.
/via Timothée Boucher
Facebook. When hovering on a new language on Facebook, they give you the name of this language in English.
/via Pierre Valade
Feedburner - You see this message after you set up your feed and want to view your Feed Stats right away.
Firefox - If you start typing a URL and you already have a tab open of that URL, the first option in the dropdown offers to switch you to that tab.
Facebook - When entering an old password, it tells you when/where you changed it, and offers you to request a new one, or try again.
pinboard.in - When changing your password, the new password text box uses a low contrast font and forgoes the typical type-your-password-again pattern.
/via Lenny Sirivong
IMDB uses names of well-known movies or actors in their Captcha when confirming a new user account.
/via Emil Björklund
Kaleidoscope - Uses their own software to compare the new licence agreement to the old one.
/screenshot - http://cl.ly/252r153E263v0R1E452y
/via David Torras