It's a gray area and for the most part ignored by all gaming companies unless the server gets too large and makes too much money. There are emulators for most large games, some active, most gone. There are a lot of World of Warcraft emulators and I am even seeing one of the advertised on YouTube now. It is most likely not worth the cost of the legal fees and publicity to shut them down unless they are actively charging people to play. The last free server I know of that was closed by a gaming company was the classic wow server that lead them to start their own vanilla server. I think they may have even hired the guys that ran it onto helping them make the vanilla wow server. There are emulators for Star Wars Galaxies, City of Heroes, Archlord, & Earth and Beyond just to name a few.