SWG Star Wars Galaxies Demo
Star Wars Galaxies like many other MMORPGs do not offer any demo of their game. The reason behind this is because the game is meant to be played online and so is designed for online play only. To have an offline demo would be like creating another game and they most likely would not put time into that.
What most MMORPGs do, instead of offering a demo to allow people to test their game, is to offer them a free time limited trial of the game where you can join the game online just as if you had purchased the retail version. Usually the the time trial lasts for a week or a month, and after that to continue playing your character you need to buy the game or purchase a subscription. The only downside to this method is that the trial version is the same size as the retail version which normally comes on CD's, so downloading 600 MB - 1.2GB for a demo may not go so well with some people.
Star Wars Galaxies currently offers a 14-day free trial of their game.