If you have an old HP plotter, you will find that the Mac OS X drivers available from HP’s support site can not be installed because they are in an old VISE .hqx format installer.
If you go and get a generic PPD for the printer, and stick it gzipped in
/Library/Printers/PPDs/Contents/Resources then you will be able to select it as a printer type when you add the printer.
Attached is the one I found and used. It needs more work, I am getting postscript errors at the end of prints which can use a lot of paper. But it is working.
Update: It looks like you can disable postscript error printing with…
defaults write com.apple.print DoPostScriptErrorHandler NO
…and re-enable it with…
defaults delete com.apple.print DoPostScriptErrorHandler
Update2: The defaults thing does not appear to work.
Update3: Using HP JetDirect as the protocol instead of LPR seems to have fixed the PostScript error problem.