Must haves:
-Pocket medicine. Not to disagree with the above poster, but I love this book. Everyone in medicine (students, interns, residents, even the occasional fellow/attending) seems to keep this in their pocket
-If you are really going computer-free: Tarascon's pharmacopeia (sp?) and a printout of the Walmart $4 formulary
-Sanford Abx guide
Rotation specific:
-Peds: Harriet Lane is big, and probably way over the top for a student, but it is a must have for peds residents
-Medicine: Pocket medicine
-Surgery: Surgical Recall (but this is more for your learning than for a general reference)
-OB: There is this thick, squat book (Obstetrics, Gynecology, Infertility) with a blue/red cover that is pretty nice, but very detailed. Probably more an intern-level book than a student level book. I got one for free but more or less never used it.