Rice Burg, which offers rice patties as buns for their sandwiches. I stopped by the truck recently, when they were parked near the Back Bay T station. Their menu is small, offering four sandwiches and sides, including rice cake skewers, fish/beef balls, and fries. Though the sandwiches are listed under Rice Burgers ($6.75-$7.75), they don't sell hamburgers and what they do sell really doesn't qualify as a burger. Instead, you'll find crispy chicken, shaved steak and shaved Shitake mushrooms. The Sides cost $2-$3.
Once I try some of their other items, I'll report back. For now, I highly recommend you check out their Crispy Chicken Rice Burger.