On my website, I want to use several logos as icons next to files whose mimetypes are associated with those logos. Most of the logos are Apple and Microsoft. Can anyone provide a source for the claim that logos can be freely used in certain contexts?
For Microsoft, I found that logos cannot be modified in certain ways, but it fails to say whether logos can be used as-is.
You're not likely to find much of a direct statement from companies about this, because they don't have to say anything. Your question is covered by the base trademark laws and fair use. (At least in the US. Other countries may have different arrangements, or no such concept at all, but that starts moving out of our scope and requires your own research if applicable.) Plagiarism Today has a decent overview of what trademark is intended to cover(and hence what you're not/allowed to do with logos), though it's combined with copyright, which people may confuse it with. The short version is that as long as you're not using the trademark with the intent of confusing or misleading people, such as implying that Microsoft endorses you somehow, you're generally safe.
Incidentally, though, from the Microsoft page you already linked in your question:
However, no license is required to use Microsoft names and trademarks to accurately refer to Microsoft software and services.
All it seems you'll be doing with the Powerpoint icon, as example is to indicate "this is a Powerpoint file." Go nuts. You have the entire web already doing it as backup if Microsoft decides to sue you. (They won't. They don't care.)
You should really contact the copyright/trademark holder (as images they are subject to copyright) and ask their permission.
It may well be that they have a page on their website with suitable icons you can download and use, but you should not assume that any image you pull off their website (for example) is free to use.
What you need to be aware of is not only the copyright of the image, but what the use of that image implies. Does it imply that your product is approved by Microsoft of Apple? If that's the case then you will have to go through some sort of procedure to be allowed to use them.
