Free PSAT Practice - https://www.collegeboard.org/psat-nmsqt/preparation/practice-for-the-test - (Practice Questions for Critical Reading, Math, and Writing)
Free 2014 PSAT Practice Guide with questions and explanations (from Collegeboard.org)
Free SAT Practice - http://sat.collegeboard.org/practice - (Question of the Day, Sample Practice Questions, Practice Tests and Tutorials from Khan Academy)
Free ACT Practice - http://www.actstudent.org/testprep - (Question of the Day & Practice Questions)