Bibliographical databases

MathSciNet Mathematical Reviews on the Web

Zentrallblatt fur Mathematik

Science Citation Index

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
Mathematics Subject Classifications
Serials and Journals - covered by Zentralblatt MATH - searchable database
Abbreviations of Mathematical Serials names PDF-file from AMS
The MacTutor History of Mathematics archive