Not sure what the formal rules are, but this is what I do.If the link to the other site advances the discussion of something I'll put the link in. Like if we were talking about X person on Y show. Let's say website Z has some info about that person...then I would post the link.
Just to go "hey look what I found" I wouldn't. Most people here know how to use search engines and if they want random websites about something they can find them just fine without me.
If the post would in any way look like I was promoting another website, I would not post it. It is not polite to spam.
Over on the Starting Over boards there were problems with people posting just to say come look at this website over and over. So on that section of the board there is a specific thread to put the information about other websites on.