The small version of traditional playing cards with notes and rhythms on the back ranging from 16th note/rest up to whole note/rest.
Playing cards can be a great manipulative to use in a music classroom. Pull random cards to practice sight reading non-standard notation (chanting the numbers or suits), allow students to arrange cards to create short compositions, use them as an inspirational spring board to begin the creative process in composition or story telling. Of course you can use them as traditional playing cards as well as typical manipulatives in a math classroom as well.
The rhythm on the back is simply an added bonus. Many standard card games like "War" and "Go Fish" can be played with rhythm cards.