Was responding to another thread and recalled that when you order a sandwich at Falafel King on Winter Street, almost always you'll get handed a falafel dipped in hummus while your sandwich is prepared...yum!
Also, the apparent trend at the Brookline Pomodoro to give everyone a free dessert. (At least it seems to happen with some regularity).
Also, I don't think they still do it, but Namaskar in Davis used to start everyone off with a little non-alcoholic sweet appertif that had threads of saffron in it.
So free, sampled, customarily comped - where do you get free goodies at places in Boston?
(I guess the amuse bouche genre counts could be mentioned too, although I am thinking something that is not necessarily advertised/expected. Bar snacks I think definitely qualify too.)