DermNet is an online source of over 23,000 images of skin disease.
ECG Library is an online collection of realistic-looking recordings that can help clinicians improve their ECG skills. It contains detailed information about how to recognize and interpret abnormal ECG findings.
Wheeless' Textbook of Orthopaedics is the largest, most comprehensive online medical textbook on orthopedics with 11,000 pages of text and over 5,000 images.