The ADS is a NASA-funded project that contains bibliographic records in astronomy and physics from both peer-reviewed and non-peer-reviewed sources. ADS consists of four databases: Astronomy and Astrophysics, Instrumentation, Physics and Geophysics, and preprints in Astronomy.
Web of Science is the world's largest, high-quality interdisciplinary database back to 1965. All subject areas are covered, but the sciences are covered very well. WoK is easy to use, but it has many advanced features that make it an excellent tool for advanced researchers. See the Web of Science page for searching instructions.
You can search all these databases, or select only one.
Web of Science (Arts & Humanities Index, Science Citation Index, Social Sciences Index)
BIOSIS (the leading biological sciences database)
Current Contents (current awareness database covering more than 7,000 journals)