If it's spam, flag it. As per What are the “spam” and “rude or abusive” (offensive) flags, and how do they work?What are the “spam” and “rude or abusive” (offensive) flags, and how do they work?:
A post should be marked as spam ONLY when it contains an unsolicited advertisement.
As for all non-spam links, I don't see a reason to depart from established protocol here:
Limits for self-promotion in answersLimits for self-promotion in answers
Post good, relevant answers, and if they happen to be about your product, so be it. However you must disclose your affiliation with the product in your answers.
There's no reason why this can't apply to links as well. However, the answers must be able to stand on their own - that is, the answer, even if not all the reasoning for it, must be able to be understood without having to refer to the content in the link. For example:
Why is the sky blue?
Bad: See this link
Good: The UCR site explains this well:
A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.
For more information, see this link: link
