I thought I have written about Greasemonkey earlier, but apparently I haven't, so here's a link to a primer in this month's Wired.
"Greasemonkey is an extension for the Firefox browser that lets Boodman's JavaScript - or anyone else's - alter a Web page as it's downloaded. The site serves the same old data, but you get to decide what Firefox displays. Most scripts fix buggy page designs or filter unwanted content such as ads and sidebar links. The best mods mix content from multiple sites, upsetting the carefully calibrated sales environments at big online retailers."
