The Verge - The logo rotates 90 degrees on hover to indicate “Back to top.”
GitHub - Hovering over relative timestamps shows a tooltip with the absolute timestamp (UTC).
Facebook. When hovering on a new language on Facebook, they give you the name of this language in English.
/via Pierre Valade
Twitter - When moving the mouse over a tweet, the name fades out to show the performable actions.
/via Benjamin Trinczek
Google Analytics - Hover over a cluster of location points, it shows a magnified version where you can select a single city
/via Mike Altman
Path - Shows a smiley on the pressed state of its signup button.