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I've seen Build and strengthen the Stack Exchange community with "crossover questions" between sites - Meta Stack Exchange, but I'm just wondering about a more limited use case; it's probably been discussed earlier, but I cannot find a reference.

I much enjoy the "Linked" and "Related" sections on the right side of StackExchange sites. Also, I often find myself finding related stuff on, say, AskUbuntu, and then adding a comment with "Related: [link]" to a related post on, say, Unix SE.

Since the engine apparently already parses URL's per site locally, so that it can create the "Linked" section, it could probably be adapted to match cross-site links too; all I'd want from this, is a small "Cross links/Linked: 3 sites, 5 questions" summary link to the right (below "Linked" and "Related" sections), and clicking on that link would show you a list of links (both incoming and outgoing, just as it works locally now).

How possible is this to implement - or maybe such a feature exists already?