Pretty good phone app while on the go – USMLE Step 1 Mastery Review

While questions aren't up to boards level, it does target information you might see there. Definitely good for doing recall questions to determine whether you actually know your stuff and is a fairly quick read. App is organized as well and you can separate out questions you don't know well.
Review by Vgthk on USMLE Step 1 Mastery.