NS3:
+ onClick, onMouseOver and onMouseOut work like always. but wait!

NS4.5/Unix:
+ onMouseDown and onMouseUp also work, so now drag+drop can be done.
+ onClick is no longer executed when onMouseUp returns false.
+ within onMouseDown and onMouseUp "event.modifiers" is defined telling
  you if the user held shift, alt and/or ctrl pressed.

IE5:
+ even onMouseMove and onKey* are triggered, but event.modifiers
  is undefined. I suppose you use some other syntax for IE.