Rosen's Emergency Medicine (need to update to the 2006 edition)
Tintinalli's Emergency Medicine
Chou's EKG Book (awesome)
Goldfrank's Medical Toxicology text
Ma's Ultrasound text
River's Emergency Medicine review text
Those are the main ones. I also have texts on wound management, billing, EMS administration, etc.
One of the better things I recently purchased was MedChallenger EM. I highly recommend it. It's a great question/answer CD-ROM/online with more than 4,000 questions.