Modal realism, usually associated with its main proponent David Lewis, is a philosophy whose credo is that all possible worlds are as real as the one we live in and actuality is a purely indexical property. Which is to say that all we mean when we call our world actual is that we live in it— the talking donkey over in possible world #3482 could make the same claim and be just as right as we are. The major modal realist text is Lewis's exhaustive On the Plurality of Worlds.
Modal realism is typically criticized as an affront to common sense, though some would have you adopt it anyway purely on the basis of its utility. It's up to you to decide whether this is a good deal or not.
For something rather oppositional, try actualism.