I'd like to start a post, hopefully with others chiming in, of fantasy or sci-fi influences, so that perhaps the playerbase can turn each other on to works that each have found engaging. Books, in particular, were the subject of thought here, though I imagine movies, TV shows, graphic novels and net-comics could all be included.
For Credible Threat:
Firstmost, the Dune series by Frank Herbert. Extremely rich, politically engaging and very vivid.
The Nehwon Series, by Fritz Leiber. The tales of Fafhrd and the Grey Mouser are light but well rendered and always entertaining.
The Thieves World Series, editted by Robert Aspirin. A fantastic compilation of author's given a setting and allowed to play with each other's lead character's, it makes for a wonderful melange of styles and visions.
The Baroque Series, by Neal Stephenson. Fabulous re-imagining of history set in a burgeoning time of technological birth and growth of thought, with Jack Shaftoe and his swashbuckling running through its entirety. One of the most interesting and lively created characters I have ever had the pleasure to read of.
Keith Laumer's Retief Series. A cock-sure, glib and sardonic interstellar diplomat, cutting through the red tape, usually with a right cross. Very funny.
And many others, including the Wheel of Time series, RE Howard's Conan and Cormac series', Michael Moorcock's Elric series, the Sprawl series of William Gibson, The Difference Engine of Gibson and Bruce Sterling (What if the computer was fully realized in the Age of Steam?), and The Ringworld series of Niven and the Amber series of Zelazny.