Mental Math

From a quick google,

http://arithmetic.zetamac.com/

This site seems to be what you are looking for, a mental math game with adjustable levels.

@veggiesushi - what's the level of your students, and what are the goals of honing mental math? I agree with the recommendation for games, and I would add computer games to it, like Wuzzits or Dragon Box. One mental math activity I love is guided visualization in geometry. Basically, you imagine geometric manipulations by description. Visual algebra works as well.

I think that the best way to build mental math fluency is to play math games. 24-Game and Math Dice are two good commercial games, but you can also make your own. I keep a collection of dice (6-sided, 8-sided, 12-sided, 20-sided), playing cards, and number tiles, and I invent games that fit the level of the child I'm working with. For a young child it can be a nice challenge to draw 4 cards and choose 2 of them to make a number as close as possible to a target number. So, if the child drew 4, 5, 1, 8 and the target number is 50, then he can make 45 or 51, and to choose between the two solutions, the child must figure out which is closer to 50. For an older child learning fractions, he might use the same four cards (4, 5, 1, 8) to make an inequality of fractions, such as 1/4 < 5/8. The goal could be to arrange the cards to make two fractions whose difference is as small as possible. Good luck!