Times Table Game (Free): Fully customisable for different tables.
Mysteriez: Hidden Numbers. A mix of numbers and Roman numerals hidden in scenes. KS2
Maths for Children: 3-5 A nice, very young child friend app. :)
Maths Workout: Overall Maths
Simply Fractions: Nice for fractions of a shape.
Zeus vs Monsters: Nice game, levels are fully customisable.
Math Claw Machine: A nice little math based game. Adaptable for different levels. Might not hold interest for long.
Maths Duel. My Year 3 love playing this - but do tend to just guess!
Controls can be a little difficult to get your head around, but training option is good.
Math Master Bingo - Easy to use, only one player.