Quincy's Reports: Reports of Cases Argued and Adjudged in the Superior Court of Judicature of the Province of Massachusetts Bay Between 1761 and 1772. By Josiah Quincy, Junior. Printed From His Original Manuscripts In The Possession Of His Son, Josiah Quincy, And Edited By His Great-Grandson, Samuel M. Quincy. With an Appendix Upon the Writs of Assistance.
Thacher's Criminal Cases: Reports of Criminal Cases Tried in the Municipal Court of the City of Boston before Peter Oxenbridge Thacher, Judge of that Court from 1823-1843.
Cases 1804-1921 Early Massachusetts Reports, Google Books
Most volumes between v.3 and v.238 are available. These are scans of the print volumes, so you know the content is accurate, but they can be cumbersome to use.
Cases 1938-date (beginning with: v.300 Mass. and v.1 Mass. App.), Mass. Trial Court Law Libraries
Cases available by citation, name, or through a Google site search. Site also includes thousands of often-cited earlier cases, and links to briefs and oral arguments.
Cases 2001-date (including Rule 1:28 decisions beginning in February 2008), Reporter of Decisions via Lexis.
Cases don't include page numbers, but this site calls itself "official" and offers the quickest access to corrected cases.
Newest Cases (Slip Opinions)
Includes opinions from the last two weeks, including Rule 1:28 unpublished decisions, available within minutes of their release. Also provides a selective e-mail service through which you can register for cases on selected topics to be sent to you.