• National Institute of Mental Health
  Information about many mental health conditions, including ADHD, Anxiety Disorders, OCD, Autism Spectrum Disorders, Bipolar, Borderline Personality Disorder, Eating Disorders, Depression, Schizophrenia (1.866.615.6464)
  
• Kids Health
  A website with information on many mental health topics, including depression, anxiety, cutting, seasonal affective disorder, suicide, and more.  There is also some general information about talking to parents, food and fitness, drugs and alcohol, school and jobs, etc.
  
• The Health Center
  There is a section on teen mental health issues including depression, anxiety, social anxiety, mood swings, bipolar/manic, depression, etc.
  
• Children, Youth and Women's Health Services
  Click on either the teen or young adult link, then the link to "Healthy Mind"

ADHD

• CHADD
  Children and Adults with ADHD
• My ADHD
  An ADHD Toolbox

Anxiety

• Anxiety Disorders Association of America
  This website has some good information regarding anxiety and anxeity disorders in teens.  The website also includes "Teens! Take an Anxiety Disorders Self Test".

Depression

• Teen Depression
  This site has links to causes, warning signs and prevention of depression in teens.  It also includes information about suicide risks and prevention.
• Psychology Info
  Some general information about major depressive disorder, dysthymia, bipolar disorder, seasonal affective disorder and others.

Obsessive Compulsive Disorder (OCD)

• Child Development Info
  Information from the Child Develpment Institute
• National Institute of Mental Health