Directories of Open Access Publications

Google Scholar
Google Scholar enables you to search specifically for references to (NOT full text) scholarly literature, including peer-reviewed papers, theses, books, preprints, abstracts and technical reports from all broad areas of research. Use Google Scholar to find articles from a wide variety of academic publishers, professional societies, preprint repositories and universities, as well as scholarly articles available across the web.  NOTE: Best used for graduate-level research.

DOAJ - Directory of Open Access Journals
This service covers free, full text, quality controlled scientific and scholarly journals.  We aim to cover all subjects and languages.  There are now 1,485 journals in the directory.  Currently 379 journals are searchable at article level.  As of today 70,244 articles in the DOAJ service.  Sponsored by Lund University (Sweden).

Free Medical Journals
Over 1,400 important medical journals available online, free and in full text.

HighWire Press
Over 15 million articles from over 4,500 PubMed journals, including 841,437 free full text articles from 847 HighWire-hosted journals.

PubMed Central (National Library of Medicine)
PubMed Central (PMC) is the U.S. National Library of Medicine's digital archive of life sciences journal literature.  Access to the full text of articles in PMC is free, except where a journal requires a subscription for access to recent articles.