A GNU program should not recommend use of any non-free program. We can't stop some people from writing proprietary programs, or stop other people from using them, but we can and should refuse to advertise them to new potential customers. Proprietary software is a social and ethical problem, and the point of GNU is to solve that problem.
The GNU definition of free software is found on the GNU web site at http://www.gnu.org/philosophy/free-sw.html. A list of important licenses and whether they qualify as free is in http://www.gnu.org/licenses/license-list.html. The terms “free” and “non-free”, used in this document, refer to that definition. If it is not clear whether a license qualifies as free under this definition, please ask the GNU Project by writing to [email protected]. We will answer, and if the license is an important one, we will add it to the list.
When a non-free program or system is well known, you can mention it in passing—that is harmless, since users who might want to use it probably already know about it. For instance, it is fine to explain how to build your package on top of some widely used non-free operating system, or how to use it together with some widely used non-free program.
However, you should give only the necessary information to help those who already use the non-free program to use your program with it—don't give, or refer to, any further information about the proprietary program, and don't imply that the proprietary program enhances your program, or that its existence is in any way a good thing. The goal should be that people already using the proprietary program will get the advice they need about how to use your free program with it, while people who don't already use the proprietary program will not see anything to lead them to take an interest in it.
If a non-free program or system is obscure in your program's domain, your program should not mention or support it at all, since doing so would tend to popularize the non-free program more than it popularizes your program. (You cannot hope to find many additional users among the users of Foobar if the users of Foobar are few.)
Sometimes a program is free software in itself but depends on a non-free platform in order to run. For instance, many Java programs depend on Sun's Java implementation, and won't run on the GNU Java Compiler (which does not yet have all the features) or won't run with the GNU Java libraries. To recommend that program is inherently to recommend the non-free platform as well; if you should not do the latter, then don't do the former.
A GNU package should not refer the user to any non-free documentation for free software. Free documentation that can be included in free operating systems is essential for completing the GNU system, or any free operating system, so it is a major focus of the GNU Project; to recommend use of documentation that we are not allowed to use in GNU would weaken the impetus for the community to produce documentation that we can include. So GNU packages should never recommend non-free documentation.
By contrast, it is ok to refer to journal articles and textbooks in the comments of a program for explanation of how it functions, even though they be non-free. This is because we don't include such things in the GNU system even if we are allowed to–they are outside the scope of an operating system project.
Referring to a web site that describes or recommends a non-free program is in effect promoting that software, so please do not make links (or mention by name) web sites that contain such material. This policy is relevant particularly for the web pages for a GNU package.
Following links from nearly any web site can lead to non-free software; this is an inescapable aspect of the nature of the web, and in itself is no objection to linking to a site. As long as the site does not itself recommend a non-free program, there is no need be concerned about the sites it links to for other reasons.
Thus, for example, you should not make a link to AT&T's web site, because that recommends AT&T's non-free software packages; you should not make a link to a site that links to AT&T's site saying it is a place to get a non-free program; but if a site you want to link to refers to AT&T's web site in some other context (such as long-distance telephone service), that is not a problem.