We generally don't allow it unless the link is direct and in context to an ongoing discussion. No links to homepages of competitive sites. If we allowed that every tom, dick and let's not forget harry would come here and start threads "hey look at my new forum site, blah blah blah...". Not constructive.

However links to real relevant sources outside of RCU are permitted and always have been and this includes say a link to a specific thread or post somewhere else or whatever.