Open Access Button - links you to free, legal, full text articles. You can search from their website or their browser extension. When you're on the page of a paywalled article, click the extension and it will look for an open access version of the article. And if it can't find one, it makes it easy to request a copy from the author.
Unpaywall - the browser extension Unpaywall, for Firefox and Chrome, adds a green tab beside research articles that you can read for free.