I have a couple: Speed's Famous Hot Dog Wagon in Newmarket Square, the sausage cart on Van Ness Street during Sox games (the one with sriracha as a condiment option), Sami's, a great but now bygone (I think) Lebanese or Syrian truck near Beth Israel which was open 24 hours: you always saw cops in that line, generally a good sign in my book.
Boston isn't a great street-food town, I guess. The lunch trucks near M.I.T. are pretty uniformly gross, in my experience. Downtown Crossing has a few burrito, sausage and hot dog stands, most of which I've thought were just so-so.
There's plenty of stuff dished out indoors that is typically street food overseas (banh mi and pho joints, the Shanghainese snacks at New Shanghai). Some of our indoor food courts remind me of outdoor food stall clusters like you see in Singapore: most of the Allston Super 88 food court is excellent, as is some of the 48 Winter Street food alley (notably Falafel King). Likewise, the super-fab Chacarero moved from a cart to a window to a big indoor space with seating. Pita Kabob comes close, but it's in a bright, clean indoor space now with a counter you can stand at.
Those don't count: I want something along the lines of an East L.A. taco truck or a Hanoi banh mi stand, places that force you to eat outside standing up, or with a flimsy paper plate resting on a car hood or the nearest newspaper dispenser.