I use three or four different kinds of cards in my job teaching English.
The most popular one is my BBC English Game (above). It was originally a board game similar to Trivial Pursuit but the board was lost years ago. Despite that, it can still be used in several different ways. For pair-work, group work, as a game, for older children, for student use at home.
The box of cards is divided into categories which encompass slang, idioms, grammar, pronunciation, vocabulary. The question is on one side and the answer is on the other. At one stage almost all my students bought these. These are immensely popular but unfortunately no longer available. If anyone has one they don’t want, please contact me! I now use them in class or loan the box to students for a term at a time.
The second most popular ones are Comcards (above). You can get them in several different languages but I use the Swedish-English ones as general conversation topics. People who are shy tend to feel more confident with a prompt, and something to hold in their hand.
The third one I made myself. This useful little box comes from Canterbury Language Training and is used for students to increase their vocabulary. I write conversation topics on the cards and can use them whenever I like. Sometimes I cut out pictures of objects and stick them to the little cards.
A normal set of playing cards can have photographs or words stuck on them and customised for particular groups. It takes a while to do them but once you have them you can use them until they fall apart.