From Platypus Firefox Add-on page:
Platypus is a Firefox extension which lets you modify a Web page from your browser -- "What You See Is What You Get" -- and then save those changes as a Greasemonkey script so that they'll be repeated the next time you visit the page.
This is very practical, for instance, if you want to fix rendering errors in a site you use but have no control over (like most IE-only sites).
Is there a similar extension for for Chrome?