I saw Eternity the other night,
Ex Tempore is a time travel setting, intended to be fairly hard sf but dealing with issues of culture, politics and exploration. When time travelers find themselves stranded not just centuries from home, but in a world where their home will never ever occur, what do they do? How to deal with people from alternate timelines – and beings from alternative earths and universes? What do you do when you find yourself stranded at the gates of a technological Heaven and Hell?
In Ex Tempore history is something constantly being edited: it is erased and rewritten again and again, by beings whose motivations range from curiosity and aesthetics to grandiose schemes to reshape all of reality in their image. Whether the universe will continue to exist is a political issue, not physics.
Flaws, Virtues and Backgrounds for the Storyteller system suitable for Ex Tempore campaigns.