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
Gmail - Typing ? in Gmail (when not in a input field) gives you a cheat sheet of all available key commands.
Twitter for mac - After typing the first few letters of a word, pressing the ESC key will show a list of words that starts with those letters.
Edit: This is a native feature of OSX and works in some other apps as well.
New York Times - Interesting interaction design around hyperlinking/highlighting paragraphs and sentences. Read more here