In addition to the websites listed for you by fellow responders I wanted to add that you may wish to check with your local school district or nearby community college. In our area, both offer free or reasonably cheap SAT and ACT prep courses.
Good luck to you and your children who will be venturing off to college soon!
You can have your teen sign up for the SAT question of the day on their website. Also, check with your local library and see if they have FREE test prep classes available. Many libraries do. Here are a couple of links: