MonoBehaviour.OnMouseOver()
Description

OnMouseOver is called every frame while the mouse is over the GUIElement or Collider.

	// Fades the red component of the material to zero 
	// while the mouse is over the mesh

function OnMouseOver () { renderer.material.color -= Color(0.1, 0, 0) * Time.deltaTime; }
This function is not called on objects that belong to Ignore Raycast layer.

OnMouseOver can be a co-routine, simply use the yield statement in the function. This event is sent to all scripts attached to the Collider or GUIElement.