Beilstein (organic & organometallic compunds)

Gmelin (inorganic and organometallic compounds)

SciFinder Scholar Information

BIOSIS Previews (includes biochemistry)

Medline/PubMed

Polymers: A Property Database

Web of Science

Journal Citation Reports [JCR on the web] - - offers a quantitative measure of the value of journals within specific subject categories.

Cambridge Structural Database -- World repository of small molecule crystal structures.

CrossFire Beilstein Information -- compilation of chemical facts.

SciFinder Scholar Information -- comprehensive chemical and related scientific information.

eEROS -- Electronic Encyclopedia of Reagents for Organic Synthesis.

Fieser and Fieser's Reagents for Organic Synthesis

CHEMnetBase (CRC)

Methods in Organic Synthesis (RSC)

Natural Product Updates (RSC)

Organic Syntheses (Wiley/CambridgeSoft; needs ChemDraw plugin)

Properties of Organic Compounds

Science of Synthesis:
Houben–Weyl Methods of Molecular Transformations

Handbooks & Dictionaries

ACS Directory of Graduate Research Web Edition

Chemical Properties Handbook (via knovel)

CHEMnetBase (CRC)

Combined Chemical Dictionary

Dictionary of Chemistry

EcoSal Online

Handbook of Chemistry and Physics (CRC)

Knovel -- Engineering and Scientific Handbooks

Lange's Handbook of Chemistry

Properties of Organic Compounds

Perry's Chemical Engineers' Handbook (via knovel)

Encyclopedias

Encyclopedia of Inorganic Chemistry

Kirk-Othmer Encyclopedia of Chemical Technology [4th edition]

Mark Encyclopedia of Polymer Science and Technology

Ullmann's Encyclopedia of Industrial Chemistry

Ebooks

Books 24x7 -- ebook collection

Handbook of Environmental Chemistry

Knovel -- Engineering and Scientific Handbooks

NetLibrary -- ebook collection

NCBI Bookshelf

Topics in Current Chemistry

Topics in Organometallic Chemistry

ChemSources Online

SciFinder Scholar (see an example)