Proceedings of all past ICM-s can be found here: http://www.mathunion.org/ICM
The following nlab pages list some of the main resources
http://www.ncatlab.org/nlab/show/Online+Resources -- a long list of math blogs and forums
http://www.ncatlab.org/nlab/show/math+institutions -- main institutions
http://www.ncatlab.org/nlab/show/math+archives -- preprint/journal/book/review archives
http://numdam.org is a collection of old issues of many mainly French math journals. http://www.mathnet.ru site has links to free old issues of most of the Russian math journals (and even some video lectures) in Russian and links to some non-free English versions. There is also an English mode of the site: http://www.mathnet.ru/index.phtml?&option_lang=eng. A smaller free depository of old issues of Polish math journals is http://matwbn.icm.edu.pl (click on the flag for English).
Max Planck maintains links to a very long list of journals, most of which are proprietary and usable only from their site, but the list is still useful because a sizeable fractions of links are also to free journals or some volumes of journals which are free, and those are mainly usable from all locations. The current URL is http://rzblx1.uni-regensburg.de/ezeit/fl.phtml?bibid=MPIMA&colors=3&lang=en¬ation=SA-SP
Many resources can be found at the sites of main world math institutes like ihes, mpim-bonn, Oberwolfach, msri, kitp, ictp, rims, ias, Steklov, Clay, crm Barcelona, Mittag-Leffler, Banff, Fields, Newton, ihp Paris
AMS keeps a long list of math societies throughout the world with links to their sites, which are often useful. One should also recommend more general AMS directory of links Math on the Web http://www.ams.org/mathweb/index.html.