I like a couple of little spots on Randol Mill Road in Arlington:

Juan Jalapeno's, just west of Fielder. $3.99 taco plate on Tuesdays, and it's excellent. $4.50 lunch specials weekdays. Homemade chips & salsa. Really good and cheap.

Los Jarros, just east of Cooper. A little crazy during lunch rush, but excellent food and also very cheap. They get a real mix of clientele--hospital workers, laborers, older Anglos, firemen.

Check 'em out.