Visit ACTstudent.org to register for the ACT and access free and low-cost practice tests and test prep information. You will also find info on the free ACT Profile career/major aptitude test. Act.org is also the first choice for information about the latest changes to the ACT.
Number2.com offers free online test prep sections for the ACT, SAT and GRE. Parents or other "coaches" can follow along to provide support for the student prepping alone.
Find everything SAT at the College Board's SAT site. They have study aids, registration information and help to evaluate and send your scores to colleges.
Khan Academy provides some practice questions and a complete practice test for the SAT for free. The number of practice questions is limited but they promise state-of-the-art learning tools for the new SAT by spring 2015.
Veritas Prep offers free SAT prep resources, in addition to paid prep classes.