Interesting one-way fare that just popped up ($10 plus taxes and fees). It's not great CPM-wise but it's a great cheap trip to Europe from Boston.
It only works on July 3rd/4th from Boston to Lisbon as a one-way. You'll travel on British Airways to London and onwards with TAP Portugal.
Update – works on more dates – just found Aug 21 and Sep 5,12,19.
Every Sun/Tue/Fri in October/November, parts of December. Goes to $441 from January.
Regular fares are about twice as high.