Add the Browser
Ruler to Favorites. Click on the link to test it, and drag from the rulers
to get guiding lines.
It's a favelet I've mocked up for personal use, using an old script by Dan Steinman and screenshots of the classical rulers. Only works on IE5.5+ Win. But don't worry, as the nicer browsers can use the amazing Ruler 1.1 by Centricle (via Zeldman).
And keep in mind there's life beyond the browser: Iconico's Screen Calipers will always get to work your pixel-perfect designs (even on Lynx!!)
Brought to you by Manuel (weblog en español)