http://greasemonkey.mozdev.org/
Basically user applied javascript to specific web pages.
Doesn’t sound that appealing? This is what it can do.
I currently have scripts that auto jump on the Gamespy, Gamespot and IGN inbetween page ads.
Scripts that clean up all the ads from Boing Boing, imdb, cnn, .
A script that turns all text web links into hyperlinks.
Hell, with a little effort, I might even be able to make a script that acts like a killfile for QT3.