Twitter - When moving the mouse over a tweet, the name fades out to show the performable actions.
/via Benjamin Trinczek
Zappos - After getting a 500 server error, hovering the three main options will make the dog show you what to do.
/via p4ul
Windows 7 - Hover colors of dock buttons change according to colors of app icons.
More information here:
http://www.istartedsomething.com/20081029/windows-7-ux-tidbits-color-hot-track/
/via hugocornejo
