American Institute of Stress
The American Institute of Stress is a non profit organization established in 1978 at the request of Hans Selye (the doctor who defined stress) to serve as a clearinghouse for information on all stress related subjects.

Medline Plus: Stress
Medline Plus will direct you to information to help answer health questions. Medline Plus brings together authoritative information from National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

Wikipedia (on stress)
Wikipedia (enPR: wĭ'kēpēʹdēə) is a multilingual, web-based, free content encyclopedia project.  Wikipedia is written collaboratively by volunteers from all around the world. With rare exceptions, its articles can be edited by anyone with access to the Internet, simply by clicking the edit this page link. 

HelpGuide.org (on symptoms of stress)
HelpGuide.org was started after the tragic suicide of Morgan Leslie Segal. Since then, dozens of dedicated and talented people have collaborated in the HelpGuide.org project to create a unique non-profit resource. The HelpGuide.org mission is to empower people to make educated health and lifestyle choices for themselves and their families.