Operating System: Windows 95 (Note: might work with other versions of this os.)
According to the readme, this driver covers:
HP Designjet 100
HP Designjet 500
HP Designjet 800
HP Designjet 5000
HP Designjet 5500
But I've only used it for the 500.
I wasn't able to find this on the HP website, because the version of the Deskjet on that website is only monochrome... Anyway, PDFs on the website and the README has installation instructions.