How to download journal research free 
Unpaywall
Unpaywall is a website built by Impactstory, a nonprofit working to make science more open and reusable online. They are supported by grants from the National Science Foundation and the Alfred P. Sloan Foundation. What they do is gather all the articles they can from all the open-access repositories on the internet. These are papers that have been provided by the authors or publishers for free, and thus Unpaywall is completely legal. They say they have about 50-85% of all scientific articles available in their archive. Works with Chrome or Firefox.

Kopernio
Kopernio is a free browser extension for Chrome, Firefox, and Opera that gives you one-click legal access to journal articles. It automatically searches your library journal subscriptions, open databases, PubMed and Google Scholar to find full-text PDFs.

Open Access Button
The Open Access Button does something very similar to Unpaywall, with some major differences. They search thousands of public repositories, and if the article is not in any of them they send a request to the author to make the paper publicly available with them. The more people try to find an article through them, the more requests an author gets. You can search for articles/papers directly from their page, or download their browser extension.

Library Genesis
Library Genesis is a database of over 2 million (yes, million) papers, articles, entire journals, and non-fiction books. They also have comics, fiction books, and books in many non-english languages.

Citationsy Archives
Citationsy Archives lets you search for journal articles and papers, download them, and of course cite them in your Citationsy projects.
After entering a query it searches through all published papers in the world and shows you the matches. You can then click a result to see more details, and immediately cite it from there.
It will also let you download the paper through a couple different, completely legal open access services.

Sci-Hub (link updated January 2020)
Finally, there’s Sci Hub. Science-Hub works in a completely different way than the other two: researchers, students, and other academics donate their institutional login to Schi-Hub, and when you search for a paper they download it through that account. After the articles has been downloaded they store a copy of it on their own servers. You can basically download 99% of all scientific articles and papers on SciHub. Just enter the DOI to download the papers you need for free from scihub.