Downtown

Eaton Centre has a comprehensive selection of small, cheap and chain eateries.  Several of these food outlets are located on the third and fourth levels, which you can reach by using escalators from the ground level.  If you ask for take-away, you can carry your lunch to the lovely indoor Devonian Gardens that are located in the next building, accessible via the "Plus Fifteen" (overhead covered walkway that connects the two buildings).  Admission to the gardens is free.  There you can find a bench in a quiet spot, and relax amongst the foliage.  

Another place where you can buy a take-away lunch to enjoy in the Devonian Gardens is the food court in the Lancaster Building.  While Eaton Centre is the next building to the west from the Devonian Gardens, the Lancaster Building is immediately to the east of the Devonian Gardens, in the same city block.  The food court in the Lancaster Building is on the second and third levels.  As is the case with many food courts, you can find ethnic foods that, while they are "fast," are nonetheless healthy, for example, Lebanese cuisine.   

Jonas' Restaurant is towards the west end of downtown at 937 - 6 Avenue SW.  It serves Hungarian cuisine (cabbage rolls, chicken paprikash, wiener schnitzel and beef stew).  Moderately priced.  Good value for money. 

Wikipedia also has a nice list of Calgary Budget Dining options.

Beyond downtown

Peter's Drive In : Located at 219 - 16th Avenue NE (what the TransCanada Highway becomes when it passes through Calgary), this place is a legend.  It serves delicious, generously sized burgers.  Great value for money.  It does not belong to a franchise.  There is only one Peters' Drive In.

Cafe Metro ( now closed as ofearly 2008) is located towards the south end of Calgary at 7400 Macleod Trail South.  It's in a small commercial strip mall, opposite a car dealership.  It's nothing to look at from the outside, but it serves good smoked meat, poutine, bagels, latkes, pizzas, burgers at reasonable prices.  

Pho Kim Restaurant is at 1511 Centre B Street NW.  It's just south of 16th Avenue North (TransCanada Highway), on the west side of Centre Street.  It serves reasonably priced Vietnames food, and the staff is very friendly.    

Reading

 Veteran Calgary restaurant reviewer John Gilchrist has recently (March 2008) published a new book of restaurant reviews, My Favorite Cheap Eats: Calgary, Banff, Canmore, Foothills and Beyond. It is available online through Canadian bookseller www.chapters.indigo.ca.