if you are talking about real broser extensions:
- http://www.enhanceie.com/ie/dev.asp
- http://msdn.microsoft.com/en-us/libr...87(VS.85).aspx
Tho this is not js related question then.

If it's IE behavior you are talking it's done via css normally:
Code:
body {behavior:url(styles/csshover2.htc);}
//where csshover2.htc is a js file overriding IE behavior for certain events