Who doesn’t like free? This is a great article from Elearing Inside about some options for free SAT and ACT! Khan Academy is well known of the three discusses and generally focuses on the SAT rather than the ACT. I’m a big fan of Khan Academy but that doesn’t mean the other two sites aren’t worth a look.
Union Test Prep (which I wasn’t familiar with) had a great user interface and very clear help for the ACT.
The 3rd mentioned in Prepfactory is also offers both ACT and SAT help.
Check them out and see which works best for you! Check out the article for more information!
3 Free SAT and ACT Test Prep Resources, and Why You Should Use Them