The real question is in what states is carrying a loaded non concealed gun not legal. If you are carrying non concealed can you be charged with a crime, if so what crime?
In other words the question isn't where is it allowed but rather where it is not allowed? The laws on this can be complex especially when you look at states that may not make open carry illegal but a state that allows each city to pass firearm laws, I believe Kansas is this way.
I generally look at this question on a state by state basis when I am interested in traveling in that state and I may not legally conceal carry there. Such a state if interest is CO.
It would be nice to produce a chart with this information. The best research I have found for this is currently the NRA website. However in order for me to feel confident enough to actually carry in an open manner I would need to research it beyond the NRA website.
Nevada allows open carry but some people argue that Las Vegas may attempt to charge you with disturbing the piece. I am not sure how true this is but I called the LVPD and they denied it.