Basically this site has all of the Merck Manuals available online for free, which means a lot of medical information you can use to make informed decisions. You can access two reference books for patients and caregivers, The Merck Manual Second Home Edition and The Merck Manual of Health and Aging.
The Merck Manual of Diagnosis and Therapy, The Merck Manual of Geriatrics, and the Merck Manual of Veterinarians are also available on this site, but they contain more technical information for healthcare professionals. They might be useful to look up more about a specific condition or test if you already know the basics from the "for-patients" books.
The Merck Manual Second Home Edition covers topics such as Accidents, Children's Health Issues, Fundamentals, Special Subjects, and Women's Health Issues, only to name a handful. Special Subjects included sections about alternative medicine, diseases of unknown cause, surgery, and medical decision making.
The Merck Manual of Health and Aging organizes 66 chapters into 4 sections: fundamentals of aging, caring for yourself and others, medical conditions, and even social, legal, and ethical issues. This section discusses how you can properly care for yourself, your children, and your family. The manual discusses how the body ages and what kind of care you need to give it.
These useful manuals put a lot of information at our fingertips. I looked up some symptoms I am having, and the manual provided me with direct questions to ask my doctor. I can go to my doctor's office better informed and have an idea of what to expect. If I don't understand something after my appointment, I can look up any tests or medical terms later.
Now, does this give you the information you need to run out and treat your neighbor's problems? No, these are online for research purposes, and to help you be better informed. They are not designed to be a replacement for your family doctor.
Click this link to view the Merck Manuals: http://www.merck.com/pubs