We stumbled on El Perigrino whilst looking for dinner. I don't know the address, but if you walk the town, you will find it. Its located in the hotel of the same name.
The front was all open to the streets, making it great for people-watching, and when we entered, there was a large table full of local teenagers playing cards and drinking. The decor was nothing special, as the paint was peeling and the tables looked shabby, but the smells coming out of the kitchen were wonderful.
We took a table by the open doorway, as the whole wall facing the street folded away to leave the restaurant open-air on that one side.
Karl ordered flank steak, plantains, black beans, and rice, whilst I went for the homemade spaghetti with tomatoes. We had a beer, a bottle of water, and some tea all for 77 quetzals (about $10).
No English at all was spoken, and the menu was all in Spanish, so some of the ordering was a little hit-or-miss, but it all turned out good in the end.
Although the seating area was quite big, at times you could see back into their living quarters, and it was what seemed to be typical here: just a small basement room with beds, furniture, kids and grandparents all lumped in together.