You can teach equivalent fractions and reinforce the idea of common denominators by writing various fractions on separate cards. Then create another set of cards with equivalent fractions. If the first set has 1/2, the second set must have something like 3/6 or 4/8, and so on. You decide how challenging you want to make the fractions. You can turn this into a matching game of speed where small groups of children race against each other to complete matching all the equivalent fractions.
A link to Jeopardy-style math games is provided in the references. The PowerPoint games are free and downloadable. It is more fun and less pressure on children if you let them play in groups. The teams pick categories and get a few minutes to discuss and come up with an answer. They use a bell to indicate when they are ready to answer. Just like the game on TV, if a team cannot answer within the allocated time or if the answer is wrong, the opportunity to answer goes to the other teams.
You can use a regular deck of cards to teach basic math facts. Remove the face cards. Give ace the value of one and use all the cards up to 10. Shuffle, then cut the deck in half and place the halves facing down. Now pick a card from each half and have the students add, subtract, multiply or divide the numbers. You can have the entire class participate. This is a great way to observe which pairs of numbers or operations are causing problems for the students that might require further practice.
Many websites have free flash or java-based games for elementary school children. If your classroom has a computer for each child, then this is perhaps one of the most efficient and fun ways to teach math concepts. If there are not enough computers, then you can pair children to play these games. There is a broad spectrum of computer-based games: concept, puzzles, strategy, etc. You will find links to some of these sites in the resources.