Reader Don K wrote to express his satisfaction with a focusing screen for the Nikon D3 from Brightscreen.com (also available for many other cameras). I have no personal experience with these screens, but a number of readers have emailed to ask about alternatives, so I’m passing it on as an item of interest. One speculative concern I have is metering accuracy; on Canon EOS one has to choose the correct screen in custom settings, even for Canon-supplied screens. Nikon doesn't offer such a setting (and offers only two screens for the D3), so it’s unclear if metering is affected.
Alternative focusing screens for D3 and other cameras (depiction)
My personal preference is for the Nikon D3 Type-E grid screen, for its gridlines. Note that in spite of the BH description, it does not offer on-demand grid-lines; they are etched onto the screen.