Take a look: http://www.gossamer-threads.com/scripts/resources/Links/Modifications/ and http://www.gossamer-threads.com/scripts/resources/Links/Sites_In_Action/
We can only help you if you tell us what you want to achieve! Go to those pages, get some ideas and try adding the mods. If you get stuck, or wish to create your own mods, this is the place to return to!