I use these signal cards for much more than multiple choice but that is the primary purpose of the cards. Since multiple choice questions are either A B C D or F G H J, I print the cards 2 sided (A with F on the back, B with G on the back, etc). When we go over multiple choice problems the students select their answers and after counting to three they hold up the card of their answer choice. *I do not allow them to grab a card until I count to three to eliminate students copying answers* I also print them on different colored paper so its easy to see. (A-F Bright Pink, B-G Yellow, C-H Green, F-J Blue)
Other options - You can do Yes/No or True/False questions by assigning a color to each response. I also put representations on the board and the students will decide "Function" or "Not a function" and hold up the color assigned to each response. They can also be used to group students - pass out a card to each student and tell them to form a group with three other students (all students must have a different color).
The possibilities go on and on. And I laminate the cards so they will last for years!