Rifts is a sci-fi RPG put out by Palladium. Due to a nuclear war, a lot of people's spiritual energy was released, which opened a large series of rifts, which act as inter-dimensional portals. This let large numbers of aliens onto the planet, bringing with them new technology. Magic also started working, Atlantis re-appeared, and some creatures which were previously regarded as legends showed up too. It's almost a meta-RPG, in that you can do almost anything in it, but GURPS is better if you're trying to do something weird.