FastCase, an online subscription database, has just launched the Public Library of Law, which they claim is the "largest free law library in the world." Through an interface that looks eerily like Google, you can search all US Supreme Court cases, Federal Circuit Court cases back to 1950, and state court cases back to 1997. Outstanding! They also claim to offer access to statutes and regulations, but the links for Massachusetts are actually to the General Court's site for the statutes, and to our site for regulations and court rules.
The service requires free registration and is clearly a marketing opportunity for FastCase, their paid service, but the coverage for cases is excellent and the interface easy to use.
In addition, Massachusetts cases from 1939-date are available online for no charge at Masscases.com.