ACT.org - This link will take you to ACT's free prep resources page.
Kaplan - Kaplan's free ACT prep resources
Khan Academy - Free SAT resources that can help prepare for any college admissions test.
Varsity Tutors - Free ACT diagnostic tests
The Critical Reader - Free resources to help students improve their reading skills
Erik the Red - Free resources to help students improve math skills
Union Test Prep - Free comprehensive test prep resources
March 2 Success - Testing resources provided by the US Army