The State Organization Index provides an alphabetical listing of government organizations, including commissions, departments, and bureaus.
Top-requested sites to log in to services provided by the state
This information was collected at a time when most municipalities did not have some form of online mapping presence. However, now most have an online assessor data lookup and many have some sort of online mapping: (if you have a website that should be included on this page, please notify firstname.lastname@example.org.)
Also see http://www.vgsi.com/vision/applications/parceldata/MA/Home.aspx for lists of many sites featuring online access to assessors' databases, some with maps.