What is this large brown bug with pincers?
You probably have a specimen of a stag beetle. 

In Europe we have a very big one: Lucanus Cervus 
but it seems to be absent in US.

Then I found an interesting web-site referring to the different stag beetle and mentioning the link to the European one.

In this above site, they mention you have to look into the antenna and as they are not very visible on your picture I think it would be difficult to define precisely what is the exact taxon of the one you found.

For specifically Massachussets region, I found this interesting link.

and they refers to Lucanus Cepreolus, but I would not be 100% sure it is this one as the antennas are not present on your picture.

References


http://beneficialbugs.org/bugs/Ground_Beetles/stag_beetles.htm
http://bugguide.net/node/view/3104/bgpage
http://www.insectidentification.org/insects-by-type-and-region.asp?