Both the ACT and the SAT have created free, 4-star apps for the iphone. The ACT app by ACT, Inc. offers quite a bit of functionality, including reading your scores, seeing updates about the tests, and of course, doing practice problems.
The SAT's app by the College Board, is more limited. It offers a "question of the day" for you to do and practice on a daily basis.
There are many other apps for the SAT, however, many of which you need to pay for, by 3rd party vendors. Many are specialized to one part of the SAT, such as grammar, or algebra.
Give them a try!