It it difficult to determine with 100% accuracy all that Google Scholar searches. Here is a list of known sites:

American Association for the Advancement of Science [citations]
Association for Computing Machinery Digital Library(ACM)
American Physical Society publications (APS)
ASCE (American Society of Civil Engineers)
BL Direct (British Library)
Blackwell Publishers (books)
Blackwell Synergy
Cambridge Journals
cat.inist.fr of Centre National de la Recherche Scientifique
CIAO: Columbia International Affairs Online
CSA Illumina abstracts
doi.org (Digital Object Identifier database)
DTIC (Defense Technical Information Center [U.S.])
Elsevier
ERIC [abstracts]
extenze.eps
Google Books [selected]
HeinOnline
IEEE Xplore
InformaWorld
IngentaConnect
Inspec
IOP Science
MIT Press Journals
NASA Astrophysics Data System
NASA Technical Reports
Nature.com
New England Journal of Medicine & many indiv med journals
NRC Research Press (Canada)
Open WorldCat
Oxford University Press
Palgrave Journals
Patents
Project Euclid
Project Muse
PubMed
Questia
RePEc (Research Papers in Economics)
ResearchNow (Berkeley Electronic Press)
Royal College of Psychiatrists
Royal Society Publications
SourceOECD
SPIE Digital Library
Springer
Taylor & Frances Metapress
Univ. of Chicago Press Journals
Wiley