http://www.kaptest.com/medical-prep/usmle/usmle-prep-course/step-1-qbank
This question bank is, in my opinion, best used for "icing on the cake" situations. This Qbank has a fair share of lower yield questions that might not appear on the USMLE. However, the questions were still good in helping me understand the concepts I already knew better (concepts cemented through UWorld and USMLE-Rx). The explanations were better than Rx but not as good as UWorld.
