1. Perhaps the best on-line directory to Massachusetts archive centers and research repositories (as well as around the world) is Columbia University Archives and Manuscript Collections. Guaranteed, you will not come away from this site without adding any bookmarks!
2. Other research repository links for Massachusetts include:
National Union Catalogue of Manuscripts
The former does need some explanation. The National Union Catalogue of Manuscript Collections is a very powerful research tool. Spend time learning how to search this database. You will be amazed at what subjects you can find. Here, in one central database, you can search for documents related to Massachusetts from repositories across the entire country. For example, a search for "Massachusetts probate records" delivered no less than 14 "hits".
Among them, the following citation:
Author: Dartmouth (Mass. Town)
Title: Records, 1674-1912.
Description: 2 linear ft.
Notes: Incorporated 1664; first town meeting held 1674.
Records of Selectmen (1674-1851) including minutes of town meetings, correspondence, bonds and petitions, highway and school records, and militia records; Board of Assessors' records consisting of censuses (1837, 1840), tax tables, valuations, and other documents; tax lists, supplemented by orders, bills, and receipts, of town tax collectors; records of town clerk consisting of intention of marriage records (1809-1825); records of Overseers of the Poor including correspondence, notices, resolutions, agreements, and financial records; surveys, assessments of fees for road repairs (1717-1822) and other records of highway surveyors; land surveyors' records (1710-1825) chiefly consisting of plats; voting lists and other records of registrar of voters; and records of town treasurer and various financial committees. Includes records of Bristol County, Mass., court officials, specifically registrars of deeds, judges of probate, and justices of Court of Common Pleas, including estate records, wills and inventories, and guardianship papers of probate judges and cases, depositions, decisions, bills, and jury lists, of Court of Common Pleas. Also contains papers of Ephraim Tripp (1753-1825) who served in several town offices including selectman, town clerk, and assessor, reflecting his civic duties, personal affairs as merchant and estate administrator, and service in General Court of Massachusetts (1810-1812); records of estate of Silas Gifford (1714-ca. 1800); guardianship records; and financial papers of Tripp and his son, James Tripp (b. 1794).
The citation also states the location of the material which is located at the Old Dartmouth Historical Society Whaling Museum Library (New Bedford, Mass.).
Another search for "Suffolk County, (Mass)", delivered 25 "hits", including the following:
Title: Wills and inventories of estates, Suffolk County, Massachusetts, 1628-1852.
Description: 72 microfilm reels.
Notes: Microfilm. Waltham, MA : Graphic Microfilm of New England, Inc., 1957. 72 microfilm reels : positive ; 35mm. Originals in Suffolk County Probate Court, Boston, MA. Consists of wills and estate inventories for Suffolk County, Ma. Index volumes placed periodically throughout reels; index also on M2470-M2471.
This particular collection is located at The Winterthur Library: Joseph Downs Collection of Manuscripts and Printed Ephemera, Winterthur, Delaware.
What researcher would have thought Massachusetts probate records would be located in Delaware?
Like we said, the NUCMC is a very powerful research tool. Records from all across the United States can be searched. Learn it, use it!