Outside the trilogy of trilogies there are five other games that bear the Ultima name. Not all of them are even set in the Ultima world.
There were two games that had "Worlds of Ultima" prefixed to their title: Savage Empires and Martian Dreams. Chronologically, they come between Ultima VI and Ultima VII (1990/91). They use the same engine as the former but are otherwise connected only very loosely with the Ultima game world.