I've been playing Guild Wars 2 a lot, and two of the player races are aliens who actually crash-landed on the planet. I think this is an awesome idea.
The Charr have a giant globe spaceship as their home city. They are big, cat-like beings with horns. They are very tough, and value strength over other attributes. They are constantly in a state of war, from what I can see.
The Asura are little floppy guys, who value intellect above all. They have a lot of golems, and golemancers are incredibly influential. They work together in krewes, and are far more concerned with Science (pulp science, at that), than marching off to war. Their home city is a giant cube, also a spaceship.