I recommend GURPS Aliens by Chris McCubbin to any GM looking to add some interesting alien races to his campaign. Many of the 20+ aliens included in this tome would make great additions to fantasy worlds. The background info provided on all these dudes fit together like its own jigsaw campaign world. You could seriously run an awesome alien-tastic campaign using your favorite generic or sci-fi rules and this book. Or say you're Traveller PCs manage to misjump their ship off of your campaign map. Just flip open this book and have them appear in a subsector dominated by one of these people.
Do you like cheesy sci-fi and horror flicks from the 50's? Then GURPS Atomic Horror is right up your alley. In addition to standing on its own feet as a source for a quirky B-Movie campaign, this book also makes a great campanion for the sci-fi/horror rpg Spaceship Zero. Author Paul Elliot always struck me as one of the cooler people to hang out at RPGnet. He often went by the handle mithras over there. He's got some very cool stuff on this page.
If you haven't read E.E. "Doc" Smith's Lensman series of novels then do yourself a favor and check them out. Lensman IS space opera. It's that effin' simple. You ever heard of the Green Lantern Corps? Howzabout the Jedi Knights? Both were inspired by the Lensmen. And the GURPS adaptation is pretty darn good. I could nitpick an item or two, but I doubt I could do better myself. Golden Age scientifical fictionary adventure at its bestest.
GURPS The Prisoner is the best fan-guide-as-rpg-book I've ever seen. Author David Ladyman does an excellent job of providing a meticulous resource for fans of the seminal British TV series while avoiding the common trap of assuming that your game must adhere closely to the licensed property.
So what GURPS books are in your game library? Any particulars stand out as favorites?