There's a place on 15th Street just above Arch (103 N. 15th St) called Bubby's Brisket and Bugsy's Weiners that has a fantastic breakfast sandwich deal for just $1! I haven't tried the lunch sandwiches or any of the other breakfast items, but the egg and cheese sandwich I got today was the best I've had from a street-food type place (and probably every bit as good as an egg scrambled at the Fountain at the Four Seasons) in all my years of buying egg-and-cheese sandwiches for breakfast.
The roll was thick and of better quality that the rolls usually used for these sandwiches. The cheese tasted almost cheddar-y. For $1, plus a free cup of coffee, I doubt you could do better anywhere in the city. The coffe was good, too. Also, the two guys who run the place were very pleasant and friendly.
They also serve cheeseteak for breakfast, and you can get sausage, Canadian bacon, steak or chicken on your sandwich, with American, Provolone or Cheddar, or mushrooms if you want. Plus, they have home fries and fresh-squeezed orange juice.
The stand has been there for about 7 months. This is an absolutely unsolicited testimonial. The place is great and deserves to be known. If you pass the area on the way to work, you should check it out. Here's a link to their site: http://www.bubbysbrisket.com/main.htm