A few notables...
MetaMemes: the What, Why, How, Where, and Who (Four W's and a H, like "Four Weddings and a Funeral," but with a lot less Hugh Grant).
IDEO Method Cards: Learn, Look, Ask, Try.
Oblique Strategies: Over one hundred worthwhile dilemmas by Brian Eno and Peter Schmidt.
Drivers of Change: The Arup Foresight group's deck that asks the question: "What will our world be like in 2050?"
We at Influx love the simplicity inherent in cards; they're so retro yet so democratic. Today's companies are outsourcing ideation less and looking within for inspiration more. There is no need to pay, because now, everyone can play.
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