papers archives? (especially not indexed by google)  http://www.digizeitschriften.de/index.php?id=239&L=2
has many papers with free access (e.g. Inventiones Mathematicae)
but when you search with scholar.google.com it does not index this site!
Are there any other archives like this? 
Just in case let me list other archives (they are indexed by google as far as I understand).
http://projecteuclid.org 
http://www.numdam.org/?lang=fr
http://www.math.uiuc.edu/K-theory/
PS
e.g. I cannot find:
Koszul, J (1981), "Les algebres de Lie graduées de type sl (n, 1) et l'opérateur de A. Capelli", C.R. Acad. Sci. Paris (292): 139-141
Does it mean search skills are poor or it is really not available electronically? 
 A: Ulf Rehmann from Bielefeld has a comprehensive list of electronically available journals (with 318 items)
http://www.math.uni-bielefeld.de/~rehmann/DML/dml_links.html
As Chandan says, this list could perhaps be added to AMS.
A: This site from the university of Goettingen is a goldmine, especially for old issues of Journal für die reine und angewandte Mathematik ("Crelle")
A: The AMS maintains a page listing all journals available electronically:
http://www.ams.org/dmr/
From time to time I have requested them to add a journal which wasn't already there on their list; if more people do so regularly, this might become the first place to visit when looking for an old article.
A: Just for completeness I will add Axriv.
http://arxiv.org/
A: Here is another German list of online journals, with indication of those freely available:
http://rzblx1.uni-regensburg.de/ezeit/fl.phtml?bibid=UBK&colors=7&lang=de&notation=SA-SP
I find it very convenient.
I did not compare it carefully with the AMS list.
A: For the dutch literature we may search in Narcis
http://www.narcis.nl/?Language=en
A: The EuDML is an interesting project that makes mathematics published in Europe available online.
A: Within the framework of the project retro.seals.ch, scientific journals are retrodigitized and made available via internet. The project contains the following mathematical journals:


*

*Commentarii Mathematici Helvetici

*Elemente der Mathematik

*Elemente der Mathematik (Beihefte zur Zeitschrift)

*L'Enseignement Mathématique
A: Many Russian journals (at least up to 2008) are available free of charge via the portal http://www.mathnet.ru/ej.phtml?&option_lang=eng
A: You might take a look at the "Digitization Registry" (http://DigReg.MathGuide.de/) in Göttingen.
