First Aid for USMLE Step I 2008 - An ABSOLUTE essential. This book contains the basic information for the entire test. If you can memorize this entire book, you are almost guaranteed a 240+ score.
BRS Physiology - A solid review of all the physiology you need to know for the test
BRS Pathology - A good companion to the physiology book
Microbiology Made Ridiculously Simple - A good review of microbiology. It probably has much more detail than you need for the test, but combine this with First Aid's microbio, and you're golden.
Pharmcards - Probably more detail than you need for pharmacology, and does contain some errors, but overall, if you know these cards, you're set for the pharm parts of the test
There are several other potentially useful books, but if you use the ones listed here and master them, you will do very, very well.
Ace Step I Without Becoming a Hermit - another guide to books for Step I written by a HMS student
Goljan Notes - the mythical Dr. Goljan apparently runs Step I review courses. His notes and mp3 lectures are heralded by some. I listened to most of the lectures, but looking back on it, they're probably best started early and listened to in one's spare time
Feel free to comment on other resources you find helpful!