Mythic provides the essential basic service of answering yes or no questions randomly, usually with the logical answer, but also with an appropriate level of surprise. Also by asking questions, you generate random events.
Mythic Variations adds useful additional tools. The main tool is themes, which use random events to lead the story along.
Once Upon a Time & Once Upon a Time: Dark Tales together contain 150 cards that are divided up into 5 groups. The groups are characters, items, places, aspects, & events. Examples of cards include 'Queen', 'Sword', 'Palace', 'Happy', & 'An Argument'.
These cards are useful for generating random ideas like NPC conversation. Another use is for subjects or descriptions of random events.
An unusual RPG/storytelling/fiction/experience game without a Game Master, inspired by epistolary Lovecraftian horror fiction. The idea seems to be that players can play live or by writing letters, and create a communal free association psychodrama experience. (I only recently acquired it and haven't finished reading it.)
This one isn't suited for a large number of players (min. 3, max. 5), but groups of players could certainly play at the same time. To say that this game gets crazy is an understatement. Probably the best GM-less role playing game available in my humble opinion.
A feature of the first edition AD&D Dungeon Master's Guide was the Random Dungeon Generator (RDG). The generator allowed, by the rolling of dice, to generate a dungeon adventure "on the fly". A dungeon complete with passageways, rooms, treasure, monsters and other encounters could easily and randomly be constructed as the player progressed. It could be used with several people or a single player. The RDG was not included in the subsequent editions of the Dungeon Master's Guide.