Most medical students use the normal question banks, such as U World, which offers a subscription model that provides a large number of questions for a set period of time. One source I didn't have when studying for Step 1 was Lecturio, which provides a FREE question bank, as well as lectures to help improve your knowledge and, ultimately, Step 1 score.
Unique to Lecturio is that there is immediate feedback for every answer provided with each question, so that when you get a question wrong (or right) you can get immediate feedback and education on the topic.
I gave my account to some current 2nd year medical students who are studying for Step 1. Most are currently using First Aid and U World, and all were impressed with the easy to use Lecturio software that doesn't require installing software, and is accessible on multiple devices. Additionally, I was told the questions are multi-step and require both foundational knowledge and critical thinking, which is exactly what is needed when studying. One student stated he appreciated how the feedback was written and was 'less frustrating and more educational' than other programs, which made him want to keep studying when he may have normally become frustrated.
If you're just starting your Step 1 studying, or looking for some extra questions, I highly recommend checking out the free q bank and considering the Lecturio video series for a low-cost way to practice questions and learn more core content.
Note: I was approached by Lecturio to review their software, and I provided my account to current medical students to trial. I was not provided a paid account for free, and all the above are my personal thoughts and feedback from current 2nd year medical students studying for Step 1.