For example, let's say that somebody asks a poll question, the likes of which are now off-topic. Is it all right to mention say, Quora? Or do I have to say "it sounds like aura but has a Q in front of it". Would Ubuntu forums be ok for an Ubuntu question? Or is linking to other sites disallowed?
It seems to me that if someone wants to engage in a certain type of discussion on Stack Exchange that Stack Exchange isn't built for, but that another particular site welcomes and does well, directing them toward that other site is good for everyone:
It's good for the user, since they'll be sent to a good environment for their discourse.
It's good for Stack Exchange, since it reinforces the message that such discourse is not supported here.
It's good for the Internet, since it unites an interested user with a high-quality forum that welcomes that user's interests.
What's good for the Internet is also good for the user, you, and Stack Exchange, since we all exist in a broad ecosystem of content-creation of various formats and on various topics, and quality content generated elsewhere becomes a potential resource for use here. For example, brainstorming in a more free-form forum could spark an idea that inspires a Stack Exchange user to develop it into a well-constructed answer here.
It depends on what you mean by "alright" and by "mention".
There is no rule that says that the mere mention of an external site or a link to it, generally speaking, is grounds for any sanctions against the user who posted the mention or the link or warrants action against the post. However, it is possible for the mention or the link to appear in such a way that will warrant some response. For instance,
Linking or mentioning a site only to promote a product. This is spam.
Posting an answer that only says "Your question is not appropriate for this site, you should post it on [link]." This is not an answer.
Posting an answer that says "Read this [link to blog post]." This is a link-only answer.
Independent of the rules, there is also the problem of how people react to the mention of certain sites. On SO in particular some folks will downvote on sight answers that link to w3schools.com. So if you mention a site that is badly regarded by the community of users, you may expose yourself to downvotes. So some would say that in this sense, it is not alright.