Live CSS Prototyper
CSS Prototyper overlays a grid on the current page. It also allows you to edit the page's content from within your browser, simply by clicking and typing. Supports pasting, cutting, copying everything from text, images, and other page elements.
Also supports bold (ctrl+b), italic (ctrl+i), underline (ctrl+u)
v0.2 UPDATE (27th Oct. 2011):
# Fixed issue of grid not displaying on sites with heavy z-index usage.
Method credited to @thierrykoblentz - http://www.css-101.org/articles/trick-for-rapid-prototyping/
Based on @webxl prototype bookmarklet - https://gist.github.com/1232664
Problems? DM me at http://twitter.com/ianleckey