You can of course use the exams the NBME offers; I think they're like $45 a pop or so, but I haven't used them yet so I could be wrong.
I am currently using PreTest for Biochem - I wish I'd used it during my course. It is very basic, but to get questions right you must know the definitions of things. I did horrendously in Biochem, so it is perfect for me. My study buddies like the PreTest Micro and Pharm as well.
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