Run by Cornell University Library, arXiv.org provides open access to over a million e-prints in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics
CiteSeerX is a computer and information science database. Here you can find links to scholarly articles and journals about related subjects.
The Current Index to Statistics allows you to search through journals related to the fields of statistics and probability. No subscription fees are charged, but both free and paid articles are returned in searches. In each case, links to where they can be found online are provided along with the citation. CIS is set to be shut down Dec. 31, 2019, so be sure to use this resource while you can!
This is a collection of computer science technical reports from the computer science department of Virginia Tech, as well as industrial and governmental research laboratories
The dblp computer science bibliography is an online reference for major computer science publications. Although it provides access to both full-text articles and those behind a paywall, you can search for only full- text articles
MathGuide is a gateway to various scholarly resources in the field of mathematics, allowing you to sort through libraries, archives, journals, open access journals, and more.
The Collection of Computer Science Bibliographies contains more than three million references. Although the site only provides bibliographic information, it includes links to PDFs or full texts on other sites where possible. There is also an option to search on Google for the full article
This is an excellent database for finding documents, authors, and journals related to mathematics. Some of them are freely available, while others require payment.