In this blog post Tobold uses the term "play money" to indicate money that is used in games such as MMORPGs. Is this a correct term to name such money? If not, what is the correct one?
Play money, to my mind, is physical money that you use to play games with, either in a formal sense such as Monopoly money, or in an informal sense such as children playing make-believe shops and the like.
I can't recall hearing someone use the specific term game money, but I would be likely to associate the term with non-physical money used in online games, that can be used to buy and sell things within the game.
For money used within games that can then be traded for real-world money, I would more inclined to refer to either the currency name (such as Linden Dollars for Second Life) or as in-game currency (note the use of 'currency' here rather than 'money'. Currency implies money that can be traded, where money just implies something that can be used to make purchases. Again, this is entirely subjective).
In reference to the particular article, though, the author first uses the term play money in reference to Monopoly money, which is exactly the term I would naturally choose for that description. I suspect that he chose the term play money in the first place because of the Monopoly reference, and then continues using that term throughout the post for consistency. If the author were to suddenly start referring to game money or any other name, it might confuse the reader.
To answer the question, I think a lot of this is very subjective, and depends a lot on the context it is used in, and the context within which the reader understands the term. There is no definite 'wrong' or 'right' here, just nuances of understanding.