Software that purports to help you do something that's illegal or that infringes copyright, such as saving streamed audio or video for reuse without permission, is unsafe.
Software of any kind downloaded from a BitTorrent or from a Usenet binary newsgroup is unsafe.
Pirated copies or 'cracks' of commercial software, no matter where they come from, are unsafe.
That exception to this rule no longer applies.) (Some reputable websites did legitimately warn visitors who were infected with the 'DNSChanger' malware.
A web operator who tells you that you have a “virus,” or that anything else is wrong with your computer, or that you have won a prize in a contest you never entered, is trying to commit a crime with you as the victim.
Any website that prompts you to install a “codec,” “plug-in,” 'player,' 'extractor,' or “certificate” that comes from that same site, or an unknown one, is untrustworthy.
That means, in practice, that you never use software that comes from an untrustworthy source, or that does something inherently untrustworthy.