It is an add-on that will show green OA logo on articles that are available as fulltext (grey if they can´t find it). They say the green tab is on 65-85% of all scholarly articles. It is fast, free and legal and do allow us to skip the paywall. It is powered by a database of millions of author-uploaded PDF´s.
It is a nonprofit project to make science more open and reusable online. Impactstory is getting grants from the National Science Foundation and the Alfred P. Sloan Foundation.