So what are these cards? The basic answer is that they were one of the earliest player-facing narrative control tools. They allow players to make changes to scenes by adding things to them (like creatures or objects), adding more abstract elements or story elements. You can add plot twists, characters, obstacles to over come and other things through playing your cards. Each StoryPath deck has three sorts of cards: theme, tone and climax. The cards are also numbered, and you must play them in numerical order.
Depending on the type of game the group is playing, you use a different StoryPath deck. For example, the Path of Adventure deck is about more pulpy elements like cliffhangers and sudden happenings. You'll find cards like"Mysterious Glyphs" or "Wild Animals." The cards aren't oriented towards any particular genre, which is a very handy thing.
Theme cards are played by the GM, setting the theme of the evening of gaming. Players can then play tone and climax cards throughout the evening. By the time a climax card is played, the path started by the GM's theme card is closed out, or it is built upon by the next round of cards. The nice thing about playing cards to do these things is that it means that quieter players won't be overrun by the louder people at the table. It gives the narrative tools a more democratic feel.
The Whimsy Cards deck is more generic, and the cards do not have as great of an impact on play. You use these cards more for matters of tone in the game than anything else. They are also not broken out in the three types of cards, like the StoryPath cards are. You can use both types of cards in conjunction with each other.
Obviously, tools for controlling the narrative elements of a role-playing session aren't going to be for everyone, however if you are interested in this sort of thing, and want a tool that isn't tied directly to a game's mechanics, then you will want to check out the StoryPath and Whimsy Cards. I like the idea that a player can play a card and add something significant to a scene, like a dangerous new creature or a previously hidden treasure. Tools like this help to get players invested in a game, they are going to get more involved in a game where they have created bits and pieces of the game's world.
The nice thing about a tool like the StoryPath decks is that they are like training wheels for player narrative control. They are more helpful for players who are new to the concept, or who might be otherwise be too timid to try to interject their own narrative ideas. As players become well-versed in the art of narrative control, you will find that they need the cards less, and come up with their own ideas more. Until then, you will get plenty of use out of these cards.
The other benefit to using them is that they let you introduce narrative control concepts into games that don't have them, like Dungeons & Dragons or any of the Palladium role-playing games. Give tried and true games that you know how things will work a new feel.
If you didn't back the StoryPath Kickstarter, you can find these cards on RPGNow in print and PDF.