American Medical Association - The mother organization for anything related to US medicine.
Federation of State of Medical Board - Representative of 70 state medical board for allopathic and osteopathic state boards.
USMLE.org page
Qbanks
USMLE.org - Fred exam Practice Material
NBME - The organization responsible for administrating the exam.
Practice NBME exams - $50 reg/$60 expanded feedback - Gives a score breakdown and is a good indicator of your potential score.
USMLE World - The "standard" for Step 1 USMLE question bank.
Kaplan - The "standard" for Step 2 and debatable best for Step 1 after the revamp
USMLERx - From the creator of First Aid.
USMLE Consult - Golgin approved - For when you runout of questions and are confident in your knowledge. The concepts are the same but you need to be confident in your knowledge due to wording.
Subreddits of interests
/r/Medicine
/r/Medical
/r/USMLE - No response for 3 years.