I had a great burger at Hewtin's mobile truck the other day (owned and operated by Chez Pascal). Really juicy beef, good swiss cheese and a sweet tomato relish. Not cheap at $7 but came with good potato salad too and worth it for the quality of ingredients. I've heard good things about their sausages but haven't tried them myself. I gather they're setting up by the park at Smith and South Main on weekdays and the Lippitt park farmer's market on Saturday.