This has proven to be a popular page, so I thought I would re-post it at the top, with one corrected URL, and a few more applications I have run across.
The FREE Viewer(s) are:
Free PLT Viewer: HP-GL/2, PNG, TIFF, JPG
If you think you might want to convert a PLT file to a DWG or DXF file, check this out.
If you use any of these applications, let us know how it goes for you.