01 Eno, B., & Schmidt, P. (1975). Oblique Strategies. Opal. (Limited edition, boxed set of cards.).
02 Eberle, B. (1996). Scamper: Games for Imagination Development. Prufrock Press, Inc.
Ideation, creativity, brainstorming, and method cards (there are many names) are physical or digital cards which can be used in ideation sessions. There are many card sets available, and it is also common to create custom sets for particular contexts.
Depending on the set, the cards are used to promote discussion, to suggest new avenues of exploration, to structure thinking, and to spark ideas. Cards can be especially useful when the group feels stuck, or is unable to move away from familiar thinking. They can also help solve a deadlock by introducing a neutral, random arbiter: the chance factor.
Every card set comes with its own instructions. Generally, each card contains a short text, perhaps with an image, which will prompt a new approach to the work or stimulate thinking. These might be questions, analogies, patterns, or more, designed to stimulate new avenues of thought. The cards often suggest a new way to look at a problem. The Oblique Strategies  set, initially developed for musicians and other artists in 1975 by Brian Eno and Peter Schmidt, is one of the most remarkable. Each card here offers a suggestion which can range from the technical (“change instrument roles,” “mute and continue”) to the conceptual (“faced with a choice, do both,” “don’t be afraid of things because they are easy to do”) or even the very human (“get a neck massage” or “tape your mouth shut”). Any of these cards could be useful in an ideation session, so the group can decide to draw one or many, or simply keep drawing until they no longer need them.
In many cases, a list (like the SCAMPER list by Alex Osborn, developed by Roger Eberle) will fulfill the function of a card set.
Card sets can function as checklists, covering all the aspects which might be considered – so if you work through the cards, you won’t forget anything important. Checklist cards can also be used for prioritizing, by simply sorting out the most important ones, or they can form the headings around which you cluster your ideas and observations.