Another math game I've played is Sparkle. Students stand in a circle. The teacher says the spelling word and chooses the first person to start it. That person gives the first letter, then it rotates around the circle. If a student says a wrong letter, he sits down and the next person has to provide the correct letter. When a student says the last letter of that word, the next person says "sparkle" and then the NEXT person sits down. The next person begins the new word and play continues until only one person is left. Hard to describe, but the kids love it.
Points are assigned based on the slip. Some slips have you add to your score, some subtract, some add to or subtract from the other team. One even has the teams trade scores. Some involve rolling a die.
Definitely a great opportunity to work on mental math! When we played, we were working a lot with negative numbers because it seemed that all of the subtract ones were drawn first! The students really got into this game and have asked me to play it frequently. Of course, it doesn't hurt that I give the winning team a Dum-Dum ;)