ArXiv  
   Open e-print archive with over 100,000 articles in physics, 10,000 in mathematics, and 1,000 in computer science. (Formerly called xxx; free)
   http://arxiv.org/

JSTOR  
   Scans of print journals, with 10 major math journals (requires subscription).
   http://www.jstor.org/

Electronic Research Archive for Mathematics (The Jahrbuch Project)  
   A digital archive of the most important mathematical publications of the period 1868-1942 and a database based on the "Jahrbuch über die Fortschritte der Mathematik".
   http://www.emis.de/projects/JFM/

2000 Mathematics Subject Classification  
   Classification used by Math Reviews (MathSciNet) and Zentralblatt MATH (ZMATH) and other math bibliographies.
   http://e-math.ams.org/msc/

MR Lookup  
   Bibliographic data from Math Reviews without the reviews (free).
   http://www.ams.org/mrlookup

Journal Copyright Policy Gossip  
   Information from authors and publishers about the copyright policies of certain journals.
   http://modular.fas.harvard.edu/copyright/

The Electronic Library of Mathematics  
   The Electronic Library of Mathematics is offered by the European Mathematical Society (EMS) in cooperation with FIZ Karlsruhe / Zentralblatt MATH. Online journals, article collections, and monographs in electronic form: access is free.
   http://www.math.ethz.ch/EMIS/ELibM.html

The Mathematics Preprint Server  
   A permanent web archive and rapid distribution medium for research articles. Preprints can be uploaded and removed. Email alerting service.
   http://www.mathpreprints.com/

Committee on Electronic Information Communication (CEIC)  
   Part of the International Mathematical Union. Its mandate is to coordinate world-wide efforts to publish mathematical papers, journals, and other scholarly work in web form. It may recommend standards on issues related to electronic communication.
   http://www.ceic.math.ca/

Fachinformationszentrum Karlsruhe (FIZ)  
   Information services for academic and industrial research. Access to MATH Database (Zentralblatt für Mathematik), International Reviews on Mathematical Education (Zentralblatt für Didaktik der Mathematik), CompuScience and index to Springer Lecture No
   http://www.zblmath.fiz-karlsruhe.de/

Mathematics Web  
   An integrated search of Elsevier journal articles and preprints.
   http://www.mathematicsweb.org/

EMANI  
   Electronic Mathematical Archiving Network Initiative: to support the long-term electronic preservation of mathematical publications.
   http://www.emani.org/

NUMDAM  
   Digitization of old mathematics documents. Free access to bibliographical data and articles of french mathematical journals. For each journal, all the volumes published up to the year 2000 have been retro-digitized.
   http://www.numdam.org/en/

Mathematical Errata  
   A growing collection of mathematical errata, corrections, and addenda to textbooks, monographs, and journal articles.
   http://www.math.mcmaster.ca/~ohagans/errata.php

MathDiss International  
   A project sponsored by the German Research Foundation to set up an international online full-text archive for mathematical theses in LaTeX format.
   http://www.ub.uni-duisburg.de/mathdiss/eindex.html

Mathematical Theses and Dissertations  
   Index to French dissertations, links to other thesis sites.
   http://math-doc.ujf-grenoble.fr/Theses/index-en.htm

Mathematics Journals Ranked by Impact  
   ISI impact factors up to 2000.
   http://www.in-cites.com/research/2001/oct_15_2001-2.html

EMIS: Mathematical Journals  
   Full text from about 40 research journals (free).
   http://www.emis.de/journals/short_index.html

ZMATH: Zentralblatt für Mathematik  
   European database of reviews and abstracts of math publications. Requires subscription, but has limited free access.
   http://www.zblmath.fiz-karlsruhe.de/MATH/home