Project Euclid http://projecteuclid.org/
Full text access to the journal Annals of Mathematics; a limited number of other journals available through paid subscriptions to Project Euclid.
EULER http://euler.lub.lu.se/euler/engine/engine.html
EUropean Libraries and Electronic Resources in mathematical sciences. Searches books, preprints, proceedings, serials, technical reports, theses, and web-pages.
SAIL-eprints http://eprints.bo.cnr.it/
E-Print search engine. Includes some archives not indexed by other engines (see the "Data Providers" pull-down menu). Use as a guest or register and receive free targeted alerts and other personalized features.
CiteSeer.ist http://citeseer.ist.psu.edu/
E-print database of mathematics and science papers with citation information. Papers may be added by specifying the URL where their files reside.