Remix culture has hit the browser. Just as you can pimp your Scion with snap-on parts, you can modify Web sites to suit your tastes - whether the authors like it or not. The enabling technology, called Greasemonkey, was created by Aaron Boodman, a software engineer who got sick of dealing with the Web on other people's terms. "I would often encounter a Web page that didn't work the way I wanted," says Boodman, who now writes code at Google and still tinkers with the software. "And I'd think to myself, I could easily fix it if I could just run my own JavaScript in the page."

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. Greasemonkey junkies have posted more than 600 downloadable site mods, or user scripts, at www.greasemonkeyed.com.

Most scripts fix buggy page designs or filter unwanted content such as ads and sidebar links. Some are more surgical, like the one that lets you read Boing Boing at the office without the site's racier posts popping up on your monitor. The best mods mix content from multiple sites, upsetting the carefully calibrated sales environments at big online retailers. Check out Amazon's page for the new Harry Potter novel, and the Book Burro script adds a yellow sticky in the corner with up-to-the-second price quotes scraped from Barnes & Noble, Powell's Books, and other competitors. Even Boodman's employer has been modded. A script called Butler stuffs Froogle pages with links to other shopping sites. It also undoes the clever copy protection on Google Print - the company's massive archive of scanned books - so you can copy and paste copyrighted book pages to your desktop.

Mark Pilgrim, the Web developer who created Butler and has written a book on Grease­monkey scripts, says he expects an arms race between site modders and site owners. For now, a company can block a specific user script by adding counter­measure code to the page. "That's due to weaknesses in the architecture that are already known," Pilgrim says. "But ultimately it's going to be unstoppable. I have the code running on my computer and you don't." Boodman agrees that tweaking is inevitable: "A lot of users view the Web as something static that is created for them, which they can either consume or not, but have no control over. Engineers have always known this was false. The Web was designed to be open and hackable from the start."

An Amazon rep said the company accepts the fledgling tech. Greasemonkey is still too obscure to pose a threat. Google wouldn't comment. But Babak Nivi, a Boston-area venture consultant, says ecommerce won't be able to ignore Greasemonkey for long. "This little baby is going to blow up business models," he says. In other words, it's time to rewrite some old scripts.