Searchable Databases

Scitopia Central search for ASA, AGU, AIP, APS, ASCE, ASME, AES, AVS, ECS, IEEE, IOP, IUCR, OSA, SPIE, etc.
GoogleScholar
SPIRES: SLAC database
SPIN: AIP's database of Searchable Physics Information Notices.

Engineering database? Try Scitopia.
IEEE
NTIS: National Technical Information Service database
NIST: National Institute of Standards and Technology
DTIC: Access to all unclassified, unlimited citations to documents in the Defense Technical Information Center (DTIC) Scientific and Technical Information Network
SCIRUS: Comprehensive science-specific search engine
NASA Technical Reports: include research reports, journal articles, conference and meeting papers, technical videos, etc.
PubMed: Access to over 12 million MEDLINE citations back to the mid-1960's and additional life science journals
Science.Gov
WorldWide Science