Currently, we can edit a question to put in a link to related items, we can add a comment and we can suggest a duplicate when voting to close, however, I think it would be beneficial if there were a simpler way to attach links to a question to other links without closing.
I envisage a "related question" button or even "related answer" that allows you to link another post to the one you're viewing. While you can do this by editing the question, adding a comment, or posting an answer, I don't think these are necessarily appropriate when say linking old questions that are no longer editable.
The benefit of such a feature would be that users would have a recognisable way of seeing related links, they could vote on the more useful related links (thereby indicating which ones provide better assistance with the question they are viewing), and the search facility of the site could utilise the link information to hopefully improve results.
I'm not entirely convinced myself on this feature but I thought it was worthy of some discussion.
To encourage discussion, I'd like to add that we should consider the opposite as well. As in, do all the different ways you can see related questions actually make it worse rather than better. Is there a way to reduce the methods into a consolidate system that is more useful?