Nivi : Greasemonkey will blow up business models (as well as your mind):

Greasemonkey lets you mash-up websites. It lets you extend and script websites and integrate that script right into the original site as if the designers had intended it to be there. It lets you use their web site, their data, their servers, their work to serve your purpose and function. There will soon be an army of hackers enhancing every site you use. Whether that site likes it or not.