Endnote Click (previously Kopernio) is a free browser plugin / extension offered by Web of Science that finds the best available PDF of an academic article while you browse. Behind the scenes, it will search open databases and (where possible) your own university's subscriptions to find the best version of the paper for you. Here's how it can help:
Unpaywall harvests content from legal sources including repositories run by universities, governments, and scholarly societies, as well as open content hosted by publishers themselves. Note: this extension currently works best with Chrome or Firefox.
Open Access Button will search thousands of scholarly sources to link to free legal full text articles. It will also request articles from authors or start a full-text request all of which can be done via a free browser extension, website or API.
It also offers shareyourpaper.org which helps make research papers open access (legally) and helps generate extra visibility and more citations for authors.
Scite will check if an article has been supported or contrasted anywhere you read literature online.
The freeware options include: