Bibliographical databases

MathSciNet Mathematical Reviews on the Web
Zentralblatt für Mathematik
Science Citation Index
ISI Web of Knowledge
ISI Web of Knowledge Journal Citation Reports
SCOPUS
INSPEC, from 1969 up to now (physics, chemistry, informatics, science)
INSPEC Archive from 1898 to 1968 (physics, chemistry, informatics, science)
OPI - Polish Information Processing Centre

Other links

Scholar Google
Microsoft Academic Search
Mathematics Subject Classifications
Serials and Journals - covered by Zentralblatt MATH - searchable database
Abbreviations of Mathematical Serials names PDF-file from AMS
Eugene B. Dynkin Collection of Mathematics Interviews
The MacTutor History of Mathematics archive
ResearchGATE