The two areas I'm aware of for links to other sites are in the forum posts and in reviews. In the Discussion Board Guidelines the section "Linking to a site which meets any of the following criteria in a post is prohibited" is pretty clear about what is not allowed.  

I do frequently come across links in reviews to ads and provider personal sites that seem to violate the logon and the no popup window (I'm assuming to yet other sites).

Do the Review links have different rules regarding acceptable functionality? If so, where would they be stated? Didn't really see anything in either the main posting requirements page that is presented prior to the Review entry page or in the Review FAQ.

(Side note on the Shill Posts and Friday Shill Day -- seems like the Friday Shill Day should be a subsection of the general Shill Posting section.)