The Chemical Reference Resolver is an invaluable tool for quickly directing you to a publisher's webpage that contains the article you are looking for. However many of the publications are now also available on preprint servers and are accessible for free even if the final publication is behind a paywall.

To search for open access versions simply prepend your doi string with the word "oadoi" and press Enter.

Full details are in this blog post http://kovsky.net/blog/posts/oadoi-resolving.