Photo credit: TurtleDiary.com
Number Twins (developed by TurtleDiary.com) is a compelling addition game to help students think about pairs of numbers that add up to the sum of 5, 6, 7, 8, 9, or 10. Our daughter and I, both found this game to be fun!
Bonus: This game also works on developing focus, speed, and accuracy.
Disclaimer: TurtleDiary used to be free, but it looks like they are now providing some brief sample plays of their games, then charging fees.