• Access Science (McGraw-Hill Encyclopedia of Science and Technolog
• Complete Dictionary of Scientific Biography
• Encyclopedia Astronautica
• Encyclopedia of Astronomy and Astrophysics
  
• Extasolar Planets Encyclopaedia
• Handbook of Space Astronomy and Astrophysics
  
• Oxford Dictionary of Astronomy
• The Scientific American Science Desk Reference

• CDS (Centre de Donnees Astronomiques de Strasbourg) provides access (among other resources/tools) to:
  
  • SIMBAD
  • VizieR
  • Aladin
    
• CRC Handbook of Chemistry & Physics
• Knovel Online (includes "Knovel Critical Tables", 2nd ed.)
• NASA's Planetary Data System (PDS)
  
• NASA's Space Physics Data Facility (SPDF)
• NASA/IPAC Extragalactic Database (NED)
• NIST Physical Reference Data

• Astronomical Almanac (The standard source for eclipse data, ephemerides of the sun, moon, planets, and more)
  SEE ALSO, the annual print version of the Astronomical Almanac has complete information for each section, and is held (from 1981 to the present) in the Kresge Library.
• Astronomiae Historia / History of Astronomy
• comPADRE (Digital resources for physics & astronomy Education - from AAPT, AAS, AIP, and APS)
• Science.gov (search engine for US goverment science information and research results)
• NASA Astrobiology 
• Solar System Simulator
• Views of the Solar System
• Your Sky

Recommended Astronomy web link clearinghouses

• AstroWeb
  A large, and international in scope, library of links to astronomical and astrophysical information available on the web.  Created and updated by a committee of astronomy professionals from 7 different institutions.
• List of Astronomy Web Resources
  A large grouped set of links from MIT -Kavli Institute for Astrophysics and Space Research.
• WebStars: Astrophysics in Cyberspace
  A collection of links to web-based astronomical resources, compiled by NASA's HEASARC (High Energy Astrophysics Science Archive Research Center).