Software Mechanics
They have a plotter driver named WinLINE which supports wide range of the HPGL and HPGL/2 plotters and works under all kinds of Windows.

HP Computer Museum
A collection of old Hewlett-Packard computer hardware, software, documentation and other marketing materials from HP's early years in the computer industry.

Analog-XY-Plotter-Output-to-HPGL-Translator
Translates X-Y analog plotter output into a stream of HP-GL commands.

Softpedia
The great software download site.