Gateway/Directory

Open Access Journals Search Engine (OAJSE)
OAIster : Union catalog of millions of records representing open access resources
Google Scholar
WorldWideScience.org is a global science gateway
DOAR : Directory of Open Access repository
DOAJ : Directory of Open access journal
DOAB : Directory of Open Access Books
Open JGate: Open Acees journal gateway

ETD (Electronic theses and dissertations)

Networked Digital Library of Theses and Dissertations (NDLTD), a single point access to ETDs of major participating institutions worldwide.
Alldissertations.com: Collection of dissertations and theses from the universities all over the world.

Digital Books/Libraries

arXiv : Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics
CiteSeerX : Computer Science
InTechOpen : free E-books and Journals in science and technology
Digital library of India
Digital Book Index : Millions of e-books across the world