Taking the posting by Maxum a step further, here is the info from Mass.gov regarding the commuter discount, which covers ezpass tolls, commuter rail, and MBTA pass costs.
Wish they had that when I was looking into working in Boston, commuting in from Billerica MA, back in the late 80's. I may have stayed in MA rather than move to NY if they had it. Oh well, that was then and this is now...
Here is the link to the Commuter Discount per Mass.gov :
http://www.mass.gov/dor/individuals/...n.html#General