The DSM4-TR is published by the American Psychiatric Association and is very comprehensive in nature. The DSM4-TR is the bible of clinical diagnostic material.  If you are looking for more information on various diagnoses, this may provide you with the information you’re searching for.
Diagnosis
DSM-IV-TR: Diagnoses and Criteria

Anxiety Disorders
Answers to Your Questions About Panic Disorder
National Center for PTSD
Obsessive Compulsive Information Center

Attention-Deficit Hyperactivity Disorder
ADDA - Attention Deficit Disorder Association
Attention-Deficit Hyperactivity Disorder, NIH
Born to Explore: The Other Side of ADD/ADHD

hronic Fatigue
Chronic Fatigue Syndrome

Depression
Bipolar Disorder News - Pendulum.org
Depression and How Therapy Can Help
Depression Screening
Depression Test, Symptoms of Depression, Signs of Depression

Developmental Disorders
Asperger's Disorder
NeuroWeb
Pervasive Developmental Disorders

Dissociation and Traumatic Stress
Sidran Foundation Home Page

Eating Disorders
American Dietetic Association
Something Fishy

Personality Disorders
Mental Help Net - Personality Disorders
Personality Disorders - Focus Adolescent Counselor Services