I finally broke down and got my hands on Mongoose Traveller after debating the issue for several weeks. (I like spending money, but I'm wary about buying new RPG material after being burned on a few occasions by, frankly, rubbish games.) I shouldn't have been worried. The world generation section is worth the price of admission on its own.
I've already created an earth-sized world with an ammonia atmosphere where sex is a form of barter and the government is a slave-owning plutocracy, and a belt of asteroids riven by balkanisation and internecine warfare where each separate rocky body hosts a different militant faction. And that has set off an adventure-seed bomb in my head. Such is the genius and beauty of a good random generator.