In my search for good test preparation materials for the SAT and ACT exams, I found Barron's, which has both a free and premium option. Their test prep is customizable (love that!) and can be done on both the computer and iPad.
Excellent practice test site - Free! :) Has Educational Exams: ASVAB, ACT, SAT, etc... Computer Exams: MCSE, MCSD etc... Professional: Legal, Medical, Accounting etc... And some extras, like IQ and movie quizzes. Great Resource!