Cassie Newton was the name of a psychic teenager in the Buffy the Vampire Slayer series, who like Cassandra, predicts her own death. (Man, these Cassandras don't have much luck.)
In the Marvel Comics Universe, Professor Charles Xavier of the X Men has an evil twin sister named Cassandra Nova, who has crazy psychic powers.
In Harry Potter and the Order of the Phoenix we learn that Professor Trelawney, the divination teacher, is a descendant of Cassandra Trelawney, a famous seeress.
River Tam from the Firefly series is a lot like Cassandra; she's got crazy psychic abilities, but she speaks in such a jumble that it's hard for people to know what she's talking about.