DropSum is an elegant puzzle game by Nick Harper, who describes it as a cross between Tetris and Sudoku. Burst bubbles and form complicated chain reactions by forming groups that add up to 9. If you ever find yourself yearning to lay down some damage with the power of sums, this is as good as it gets.
Take the simple kid's game Concentration, disguise it as something even easier and more kid friendly, then make it much more challenging and evil with each passing level. Flipped out is a super-polished and all around entertaining twist on a familiar game mechanic.