Many practice driver's license tests for any state in the U.S. are available free online. A variety of tests are offered at DMV Written Test website and Driving-Tests.org.
Both of these websites explain the written test format for a given state and offer a number of practice tests that emulate the actual tests given by that state's driving authority. DMV Written Test also offers state-specific driver's handbook information about driving in each state. Driving-Tests.org offers practice-mode tests, in which test takers may request hints, as well as DMV-style tests that are timed and scored like genuine tests.
Driving-Tests.org also offers multiple choice tests meant to simulate common scenarios that applicants face during the behind-the-wheel portion of the driving test. In addition, YouTube hosts many videos explaining common driving test errors and showing DMV driving test routes and tips in a variety of locations across the United States.