DOAJ: Directory of Open Access Journals Free full-text database of journal articles, covering all subjects, but emphasizing natural and health sciences.

General Science Collection (EBSCO)--Full text coverage for more than 60 of the most popular science publications.  Provided by NOVEL

HighWire Press Produced by Stanford University Libraries, HighWire Press hosts the largest repository of free full-text life science articles in the world, with 1,668,695 articles available without subscription.

Science Reference Center (EBSCO)--Full text articles from a collection of reference books and journals relating to all branches of science. Provided by NOVEL

ScienceDirect (Elsevier)--Provides access to over 700 full-text Elsevier Science journals.