So if you are searching on Google or on Google Scholar and you see a perfect article and you don’t see a happy HTML or PDF note on the right of the results, and you cannot get to the document without access to the wealth of a university library login, give this a try. Click on the result and see if one of these little tabs floats along the right edge of the screen.
When you view a research article, you’ll see a tab on the right of your screen:
a green tab means free legal full text is available
A gray tab means the search came up empty.
Note that the Nature article here would cost $32 to read on Nature.com.
Note also: My personal testing reveal a whole lot more gray tabs than green ones and it seems that tabs appear more frequently for article in the sciences and social sciences. Not so much yet for the world of library research.
Unpaywall.org is a growing organism. Not all repositories have yet been included and it doesn’t yet pull from a few portals where researchers voluntarily upload content, like Academia.edu or ResearchGate.
Follow news and progress of the project on Twitter @unpaywall. 
Here are a couple of related options for getting full-text without access to an institutional library:
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