As the title asks, is there a spell or magic item that allows one to see ethereal creatures?
The see invisible and true seeing spells both work.
That'd work. Since the ethereal plane overlaps the material plane, is there a spell that allows one to see the ethereal plane, ala Tillinghast Device from "From Beyond"?
True Seeing sees into the Ethereal Plane if you focus on it. See invisibility can see any ethereal creatures or objects within your line of sight.
There used to be a spell and/or magic item called corpse candle. I don't know if if made it into pathfinder but basically it gave you a candle and in the candle's light radius you could see ethereal creatures.
And don't forget, See Invisible is on the list of spells that can be made permanent (until and unless dispelled) for the fairly nominal cost of a Permanency spell, at least if you are the one casting the Permanency. Of course for a non-spellcaster that's not generally possible, barring a Ring of Spell Storing, but it's a great investment for anyone who can do it.
The trait Ghost Sight lets you see Ethereal creatures as merely Invisible.
It isn't a magic item or spell, sadly ... However - everyone's favourite (totally balanced *cough*) spell Paragon Surge allows you to pick up the Extra Traits feat and thus this trait.