An example closer to home is those file sharing sites that allow you to pay to "download faster" which skirt very close to the line, if not go over it.

But Grokster et al openly advertised that you could get all music "for free".

Where the gray area is would be something that arguably can ONLY be used for piracy (an example of what this would be is hard to imagine, but maybe a device that can ONLY duplicate encrypted blurays and cannot do it for non-encrypted ones - yes I know there are arguments even here via fair use/backup/personal copy) and/or something that is substantially advertised as for piracy - something that nobody would have a reason to buy unless they were pirating, perhaps - something where free/open source similar software exists but can't be used to pirate?