Linking to other websites is encouraged, where it is both legal and ethical to do so, so long as the links are helpful and/or constructive (even if they belong to another forum like this one). If they are just sales pitches, providing little or no benefit to others, then such is discouraged. If you're doing a sales pitch, please actually say so; it's not all-out forbidden, but I may moderate away some posts, if I don't think they contribute to the site very well. Don't repost the same thing over and over again, either, please.
If a site has particularly objectional content for families or sensitive individuals, please say so (or don't post it; some links may be deleted). If the site has many authors (such as a forum), you only need to mention objectional content if it's on the specific page that you're linking to—unless the topic of the entire forum is objectional or something.