http://gateway.nlm.nih.gov/gw/Cmd

The NLM Gateway allows users to search in multiple retrieval systems at the National Library of Medicine (NLM). The current Gateway searches MEDLINE / PubMed, OLDMEDLINE, LOCATORplus, MEDLINEplus, ClinicalTrials.gov, DIRLINE, AIDS Meetings, Health Services Research Meetings, Space Life Sciences Meetings, and HSRProj