CATmaker from the Centre for Evidence Based Medicine (Oxford, UK) is a free, computer-assisted critical appraisal tool to provide guidance for critical appraisal and calculations for therapy, diagnosis, prognosis, etiology and systematic reviews.
The Cochrane Library - "a collection of six databases that contain different types of high-quality, independent evidence to inform healthcare decision-making, and a seventh database that provides information about groups in The Cochrane Collaboration."
National Guideline Clearinghouse (AHRQ) - from the U.S. Department of Health & Human Services offers summaries of evidence, clinical guidelines and more.
PubMed Health - "specializes in reviews of clinical effectiveness research, with easy-to-read summaries for consumers as well as full technical reports. Clinical effectiveness research finds answers to the question “What works?” in medical and health care."