For developer who work for PSD to XHTML Conversion jobs, i think the firefox adons pixel perfect
is a must…

About Pixel Perfect:
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.

By toggling the composition on and off, the developer can visually see how many pixels they are off in development. Pixel Perfect also has an opacity option so that you can view the HTML below the composition. By being able to see both the composition and the HTML you can now simultaneously use Firebug while Pixel Perfect is still in action.

Although we do understand that all browsers render slightly different, and sometimes even OS dependencies come into play, we still believe Pixel Perfect is a great tool to help keep the integrity of the perfectionist designer.