Math+review

http://www.interactivestuff.org/sums4fun/sequences.html [6-12] Challenging, but not impossible

http://www.spacetimearcade.com/AddUp [k-6] I think it'd be kind of hard for little kids to keep up with the rate of the decent of the balls. But I found it to be rather fun, stressful but fun :P

http://www.spacetimearcade.com/Snake [k-6] Easy, fun.