Mathboard is quite nice. It helps practice speed drills on the four basic operators (+,-,*,/) plus spuares and square roots. They have a free version that only does addition so you can try it out first.
Another one that has potential is Algebra Touch. It drills basic algebra rules, factoring, operator order of precedence, and even generates problems for you. It's quite different than Mathboard, but the two complement each other nicely.