The Obsessive Compulsive Foundation
http://www.ocfoundation.org
The OC Foundation is the largest OCD-related organization in the U.S. Its website provides information on OCD, treatment, and research. By becoming a member ($45) you will receive the OCF's regular newsletter, a booklet on living with OCD, and information about their annual conference.
The Madison Institute of Medicine
Obsessive Compulsive Information Center
http://www.miminc.org/aboutocic.html
The Obsessive Compulsive Information Center has lots of useful information on OCD. The Center has over 19,000 articles on OCD that you can search and request, for a fee. It also publishes booklets on OCD, and research packets on many different aspects of OCD (available for between $5 and $10).
National Institute of Mental Health OCD Page
The NIMH has various online publications regarding OCD.

For a general list of its publications, see:
http://www.nlm.nih.gov/medlineplus/
obsessivecompulsivedisorder.html

The NIMH has published an online booklet about OCD, its causes, and its treatment. The site also has a list of organizations, recommended books, and research articles.
http://www.nimh.nih.gov/publicat/ocd.cfm

The site is available in Spanish at:
http://www.nimh.nih.gov/publicat/spanocd.cfm

The NIMH also has an online diagnostic tool to identify if you might have OCD. It is available at:
http://www.nimh.nih.gov/anxiety/ocdri1.cfm

It also has an online screening test:
http://www.nimh.nih.gov/publicat/ocdtrt1.htm

For a general NIMH publication on OCD, see:
http://www.nimh.nih.gov/publicat/ocdsoms.cfm

OCD Action
http://www.ocdaction.org.uk/home.htm
This is an excellent British site with lots of information on OCD. It is relatively new, and packed with information. There is a good section on OCD in young people. Although the site is not designed for children, it has useful information for parents.

OCD Online
http://www.ocdonline.com
This site was created by Dr. Steven Phillipson, a psychologist in New York City. The site contains various articles on OCD by Dr. Phillipson, as well as some "success stories" of patients who were treated with cognitive-behavioral therapy.
HealthyPlace OCD Community
http://www.healthyplace.com/Communities/ocd/
sandra/ocd_children.htm

This British site has information about OCD, including about OCD in children. It allows users to post messages. It is not designed for children to use, but does have some useful information.
  The Anxiety Community
http://www.anxietyhelp.org/information/
ocd_symptoms_checklist.html

This website has some information on OCD, but most interesting is an online checklist of symptoms. You can fill out the checklist and then submit it. The site will then provide you with a report or summary of your current symptoms.

1-Obsessive-Compsulive-Disorder.com
http://www.1-obsessive-compulsive-disorder.com
This site has some information on OCD, although it is not aimed at OCD in children.  

Geonius.com
http://www.geonius.com/ocd/index.html
This site has information on OCD. It, too, is not aimed at OCD in children.