The American Psychiatric Association provides an overview of Mental Illness in this pamphlet from the Let's Talk Facts series. 
Mental Help Net — MHN offers summarized versions of official diagnostic criteria, catagorized byadult, child, and personality disorders.  A growing list of self-help questionnaires can be used to help determine if further professional mental health assistance is needed. 
Public Information: The Insanity Defense  —  What it is and is not.
Call Me Crazy  — An interesting article about the political aspects of the development of the classification of mental disorder. 
Dr. Bob's mental health links
Dual Diagnosis Website
HealthAtoZ.com  —  Allows you to search over 50,000 professionally reviewed health & medical Internet resources.  The Mental Health page provides links to nearly 175 professional- and consumer-oriented sites.  The number of links on the Disorders and Conditions page is approaching 100., and there are links to nearly two dozen pages of specific catagories.  A MegaPsych GottaSee site.
Internet Addiction Disorder (I.A.D.)
Internet Mental Health
Internet Mental Health Resources
Online Psychological Services - Mental Health Info Link
Mental Health Today - Mental Health Today provides information on Borderline Personality Disorder, Bipolar Disorder, Attention Deficit Disorder, Post Traumatic Stress Disorder, Narcissistic Personality Disorder, Gender Identity Disorder and Suicide Intervention. Resources, communities, tests, free medication, articles and much more.  A MegaPsych GottaSee site.
Mental Health Info Link
Mental Health InfoSource
Psych Central: Grohol's Mental Health Page
Mental Disorders Symptoms List by John Grohol
Psychiatry On-line
Psychiatray On-line Glossary

Anxiety Disorders
The American Psychiatric Association offers information about several of the Anxiety and Avoidance disorders in the Let's Talk Facts series, including Anxiety Disorders, Panic Disorder, Phobias, Post Traumatic Stress Disorder, and Obsessive-Compulsive Disorder.  (OCD is available only in PDF format.  You will need the free Adobe Acrobat Reader to read or print it.)  Each pamphlet describes the disorder, the major symptoms, what we know about the causes, and the treatment of the disorder. 
Center for Anxiety and Stress Treatment
Obsessive-Compulsive Disorder
Panic Attacks
Panic attack - Frightening condition is not just 'nerves' or stress —  From the Mayo Clinic's Health O@sis. 
Traumatic Stress Home Page

Attention Deficit Disorder
Attention Deficit Disorder Archive
Attention Deficient Disorder Home Page
CHADD--Children and Adults with Attention Deficit Disorder

Autism
Autism FAQ
Autism Resources

Childhood Disorders
The American Psychiatric Association offers information about Childhood Disorders in the Let's Talk Facts series, describing the disorders, the major symptoms, what we know about the causes, and the treatment of the disorders. 
The Child Psychologist — Rene Thomas Folse has created a site that focuses on Specific Disorders and Other Reasons for Concern about the behavior of children, and Treatment, Resources and Remediation of some of these disorders and concerns.  You will also find links to information about several specific diagnostic tests used with children.   A MegaPsych GottaSee site.

Dementia
Alzheimer's Association
Alzheimer Web
Mayo Clinic Alzheimer's Resource Center — Take a quiz about Alzheimer's Disorder, read articles, and visit linked sites. 

Dissociative Disorders
International Society for the Study of Dissociation
Maytal Ltd.
Sidran Foundation-Dissociative Disorders
Trauma and Dissociation

Eating Disorders
The American Psychiatric Association offers information about Eating Disorders in the Let's Talk Facts series, describing the disorders, the major symptoms, what we know about the causes, and the treatment of the disorders. 
 

Enuresis
National Enuresis Society
Bed-wetting - enuresis - primary nocturnal enuresis

Mental Retardation
Mental Retardation: ARC home page
Down Syndrome WWW page

Mood (Affective) Disorders
The American Psychiatric Association offers information about Depression and Manic-Depressive/Bipolar Disorder in the Let's Talk Facts series, describing the disorders, the major symptoms, what we know about the causes, and the treatment of the disorders. 

Personality Disorders
Borderline Personality Disorder
Borderline Personality Disorder Today —   Formerly named "The Borderline Sanctuary", this site includes information on Borderline Personality Disorder.  Resources, communities, tests, free medication, articles and much more.
The Personality Disorders

Schizophrenia and Other Psychotic Disorders
Schizophrenia — This is the electronic version of one of a series of Let's Talk Facts pamphlets published by the American Psychiatric Association.  Describes schizophrenia, the major symptoms, what we know about the causes, and the treatment of schizophrenia. You can backtrack from this page and see other pamphlets in the series. 

Sexual Disorders
Pedophilia
Sexual disorders

Sleep Disorders
Circadian Learning Center

Speech Disorders
Hyperlexia Home Page

Substance Use Disorders
The American Psychiatric Association offers information about Substance Abuse in the Let's Talk Facts series.  The distinction between use and abuse of substances is described.  Substances covered include alcohol, marijuana, cocaine, opiates, hallucinogens, inhalants, sedative hypnotics, and nicotine. 
Drugs and Individual Behavior Page