That's an HP Graphics Language plotter file.
In the future, you might try googling ".extension format" or ".extension file". 99% of the time, you'll find the file format in the very first result.
There are plenty of FOSS HPGL and HPGL/2 interpreters and convertors:
In addition to these, a lot of CAD and vector graphics programs support HPGL. Though there seems to be a lack of standardization in file extensions used, including: .hpgl, .hpg, .hgl, .plt, .hp, etc.
Also, later versions of PCL seems to support HPGL as well, so this further adds to the hardware and software support for the format.
More info can be found here.
Edit: Thank you G Koe for initially appending my comment to this answer and prompting me to make a little more effort to expand it. I've community wikied this answer so other freeware and open source apps may be added, as I'm sure HPGL's support is more extensive than what's currently listed.