Also Sandy's Lunch Box in Pepperell on rt 119 (where 119 and 111 meet) has a great reputation. Owned by the same folks as the Scotch Pine Inn.
Further south of that area but still on rt 119 in Littleton is Ed's Weenies, located in the parking lot at Gary's Farm Stand.
I live in Chelmsford, and the Elliot's location at Drum Hill has been closed for a while. Replaced by a small diner. (in a strip plaza)
Not sure about Sullivan Farms on Rt 113 in Tyngsborough...they are an ice cream stand that sells hot dogs. I have never ordered them there.
Also, Sausage King on Main st. in Downtown Nashua. (near where the river intersects with Main St.) has standard ballpark dogs and 10 to 15 "Gourmet Dogs", plus all the fried ballpark-style sides.
And of course, Italian sausage with peppers and onions.