The Boston College number theory and algebraic geometry seminar is a research seminar. (Take the Red Line or CT2 bus, followed by Green Line B to Boston College or Green Line D to Chestnut Hill; then walk to Maloney Hall. Allow an hour.)
The Boston University number theory seminar is another research seminar. (The easiest way to the BU math department may be to walk. Allow about 35 minutes.)
The Tufts Algebra, Geometry, and Number Theory Seminar is not far from the Davis T Station.
The Harvard-MIT algebraic geometry seminar is a research seminar whose location alternates between Harvard and MIT.
The Harvard number theory seminar does not have a separate home page; you can see upcoming talks listed on the Harvard math department seminar listing. (Take the red line from Kendall, in which case allow 20 minutes. Or walk up Broadway, in which case allow 35 minutes.)
Informal graduate student seminars often exist at Harvard, but you may have to talk to a Harvard graduate student to find out about them.