How useful is it to edit posts and add Wikipedia-links to (probably) well-known things like PowerPoint, Mac OS X, Python or YouTube? Or to (probably) less-known things like Norton and Avast! antivirus?
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For common terms, I'd say it is not necessary - quite the contrary, it adds clutter to the edit history, and unnecessary distractions in the question's or answer's word flow.
If you don't know what Python, Mac OS X, or Youtube are, you can always look them up in Wikipedia. If you don't know how to do that, you're probably in the wrong place anyway.
I'd add links only for lesser-known terms that may need explaining even to the intended audience of the question or answer.
As useful as the articles that you are linking to are and will be in the future. I try to avoid linking to anything undergoing a serious edit war, or articles that have serious issues (i.e. not a single citation when citations are possible).
If the article in question can potentially drastically change to the point that its meaning contradicts, not supports your post, don't link to it. But then again, that goes for any external link.