I use Safari now, so I don’t get the fabulous Web Developer Firefox plugin unless I go way out of my way. There’s no replacement that works with Safari, but Allan Jardine’s Design bookmarklet does give us some nice tools.

Design is a suite of web-design and development assistive tools which can be utilised on any web-page. Encompassing utilities for grid layout, measurement and alignment, Design is a uniquely powerful JavaScript bookmarklet.