There is a new service by ImpactStory available: http://oadoi.org
This service is a special DOI resolver. The oaDOI link leads you to an open access version of the article (if available) instead of the publisher’s version behind a paywall. Just paste a DOI.
MPG/SFX already includes the oaDOI.
More about oaDOI: http://blog.impactstory.org/introducting-oadoi/