Steve Jackson Games has started taking pre-orders for a couple of new RPG books based on some rather popular properties. First off, they've got the GURPS books for both Mars Attacks and Discworld.
In Mars Attacks, pick your side as either the Earthlings or the Martians. Either work at invading and killing all the humans (but with a limited vocabulary) or trying to protect your home from invasion. Or, over in Discworld, you can jaunt across the multiverse, dealing with orcs, goblins, and ogres across various planes of existence.
So whether you want to deal with flying saucers, or just saucer-shaped worlds, they've got you covered.