Which Exam Should I Take? - this link explains the differences between the two tests.
The SAT Preparation Booklet - an 88 page booklet produced by the College Board, from 2007-2008.
NOTE: It looks like the College Board has finally come up with something to replace or supplement the old booklet. The new Getting Ready for the SAT can be found here.
Preparing for the ACT - an 80 page booklet produced by ACT.org
Free Official SAT Practice Test
Free ACT Online Demo and Practice Test
I Need a Pencil - SAT Prep. Includes "60 engaging lessons in math, reading, and writing that infuse pop culture into learning to make prep accessible," as well as practice questions and more.
Number2.com - SAT and ACT Prep. Website that "offers students access to comprehensive free online test preparation courses for the SAT & ACT."
Saylor's SAT Prep course
Khan Academy's SAT math prep videos
DOME SAT Review - SAT Prep, study guides, flashcards, practice exams, and additional resources.
MajorTests SAT Prep - Hundreds of SAT practice questions. (Arranged in quizzes. Explanations for questions you answered wrong are provided.)
SparkNotes SAT, SAT subject exams, and ACT prep
ACT Practice - scroll down on the page to find the free links
5,000 Collegiate Vocabulary Words
Vocabulary Flashcards - SAT-level words with pictures to help with memorization
SAT Games - for practice
Educator Guide to the ACT Writing Test - this explains how ACT essays are scored
SAT & ACT Test Preparation Tips and Resources - Thanks to Danielle from Colorado for this link suggestion.
You might also be interested in: Student Edge - "StudentEdge is a free online college and career planning resource center. It offers tools to help all high school students take ownership of the college planning process by letting them find scholarship money, prepare for exams, search for colleges, and explore career interests."
*Special note for military families: Active duty, retired, veterans, reserve, National Guard, DoD employees, and their families and dependents may be eligible for a free copy of SAT & ACT prep software valued at $200, thanks to a program run by MLB and NFL players. You can find out more by visiting this page at military.com.
Recommended Course of Study:
The earlier you can begin studying for the SAT, ACT, or both, the better. Make time daily, if possible, to work on building your vocabulary. (And remember, one of the best ways to increase your vocabulary outside of dedicated study of particular words is to read good literature. Pick a classic novel and as you read, write down any words that are new and unfamiliar, building a personal dictionary.) Read the link that explains the differences between the tests and decide which one you will take. Download the exam booklet you need and register at I Need a Pencil and/or Number2.com. You may want to give both sites a test run to see if one works better for you than the other. Complete the Saylor course. Keep a study journal record of what you have studied. Include vocabulary words, math concepts, and any practice quiz/test results.
Note: If you have access to a public library, you should check their test prep section for books on the exam you will be taking.