It is equally important to hold the students accountable for their time on the tools both to show mastery of the objective and to allow time for reflection.
I visited Study Ladder, ManipulaMath, TES and Thinkfinity. My favorite was Study Ladder's interactive math. I would have it as a station on the computer or active board. I would hold the student's accountable by having them complete sentence stems that I have started for them on the targeted content objectives.
For ipod apps, I tried out Basic Math, ClockMaster and Fraction Factory. ClockMaster was my favorite. and I think that I would hold the students accountable by having them list the analog times of the clock faces they read.