AMCAS

AMCAS is the overall national website to apply to medical school. Other than tips for applications, you have access to FAQs and deadlines for different medical schools. 

OldPreMeds

SDN has various forums for everyone in the medical field.

MSAR (Medical School Admission Requirements) book:

For allopathic medical schools (MD).

College Information Book (CIB)

For osteopathic medical schools (DO)

American Association of Colleges of Osteopathic Medicine (AACOM):

For osteopathic medical schools (DO)

Association of American Medical Colleges (AAMC):

For allopathic medical schools (MD).