Mouseover events are not limited to web design and are commonly used in modern GUI programming. Their existence might not even be known to the user as the events can be used to call any function and might affect only the internal workings of the program.
A special usage of mouseover event is a tooltip showing a short description of the GUI object under the pointer. The tooltip generally appears only after the mouse is held over the object for a certain amount of time.
Some websites use links that have give no visible indication of what the link points to, and in some cases, no indication at all that it's even a link. When websites do this, it's called pig in a poke or mystery meat navigation. This type of design makes such websites difficult to use since users don't know where to find the link they need.
<!-- Direct usage is not recommended. It does not conform with web standards. --> <img id="myImage" src="/images/myImage.jpg" onMouseOver="alert('your message');">
- DontClick.it: Demonstration of navigation using only mouseover (requires Flash Player)