The Cochrane Library
The Cochrane Library consists of a regularly updated collection of evidence-based databases, including over 5000 Cochrane Reviews published in The Cochrane Database of Systematic Reviews. (A brief introduction to The Cochrane Library is available here.)
These reviews explore the evidence for and against the effectiveness and appropriateness of treatments (medications, surgery, education, etc) in specific circumstances. The Cochrane Library is published monthly by Wiley Online Library and has an Impact Factor of 5.715.