ScienceDirect: Full text scientific database, covering physical, life, social and health sciences.
Scopus: Database index to articles in the sciences.
SpringerLink ejournals: Scientific, technical and medical journals, some in full text.
Web of Science - all databases: The 'all databases' section of Web of Science consists of Web of Science, Current Contents Connect and Medline. Under this link you can search across all three simultaneously.
Wiley Online Library: Full text journals in a range of disciplines. Subject coverage crosses all areas of the physical, biological, medical and applied sciences.