Amazon - When not on the Amazon homepage, hovering over the logo will make it look like a button and show the word “homepage” under it.
/via Lior Fisher
YouTube - If you are not logged in, the “respond to this video” textarea asks you to Sign in or Sign up on mouse-over.
/Via Marian Buhnici
Wolframalpha - When you move the mouse to the corner of a result it reveals useful contextual links.
Google Translate - Hovering over the translation, highlights the matching phrase in the original text. Clicking on it reveals alternate translations.
/via Shruti Ramiah
Flickr - As your cursor moves into map and into location marker, the map zooms from nation to region to street level.
TweetDeck - Shows a friendly message when you hover over what is usually the “Follow” / “Unfollow” button on your own profile.
Rdio - When you mouse over the forward/back buttons on the Rdio player, it tells you what track is coming up next in the playlist/album.
Windows 7 - Hover colors of dock buttons change according to colors of app icons.
More information here:
Chrome - Status bar shift down with the cursor when it is hovering a link near the bottom of the page. After a few seconds its hows the entire URL.