And they're easy to come up with: just take a real-world game and change it slightly. Put Chinese chess on a circular board and change the tiles. Play chess with holographic monsters. Combine Blitzkrieg with Stratego.
For a lot of fictional games, the rules don't actually matter. Although fans have made up rules for Avatar's Pai Sho and Song of Ice and Fire's cyvasse, nobody knows the rules used in the actual worlds because they don't matter. The writers have an idea of the basic concepts of the games (taken from the real-world games they combined) and they only reveal what they need to keep the plot moving.
But sometimes you want more than that. A critical event might turn on the outcome of a bet, like in Pirates 2 or Phantom Menace. Or your entire plot might center on a game, like Ender's Battle Room. In these cases, the reader needs to understand and care about what's going on. They need to know the rules.
If you're not into game design, keep things simple. Liars' Dice, podracing, and even the Battle Room are directly translated form real-world games. The writers only made slight alterations for their settings.
If you want something more complicated, be warned: an unbalanced game, whose rules are detailed in the story, will shatter the reader's disbelief. You can solve this by asking, "How could I break this game so that I win every time?" and then fix it, but that's getting into game design techniques, which I don't think you came here for.
Got that? Here's the summary:
- Fictional games are easy to make: take a real-world game and change it slightly.
- If the plot does not hinge on the outcome of a game: be vague about the rules.
- If the plot does hinge on the outcome: stick as close to the rules of a real-world game as possible.
- If the plot hinges on the outcome and you really, really want to come up with something unique: welcome to the world of game design, my friend. Here's a list of games to study up on.