Léon de Bruxelles is a chain of inexpensive to moderate brasseries.  I like it for a quick and simple meal of moules frites (mussels and fries) and other Belgian treats.  The beer is pretty good too.   It is fine for either dinner or lunch.   Website (French only): http://www.leon-de-bruxelles.fr

For a variety of inexpensive restaurants with good food and service, go to rue Grégoire de Tours between rue de Buci and Boulevard St. Germain near the Odéon métro station in the 6th,.  My favorite is Au Beaujolais at 9, rue Grégoire de Tours.  Be sure to tell Hakim that I sent you!   He's got some traditional French dishes served up with some of the best hospitality that you can find in Paris.

Pâtisserie Viennoise,  8, rue de l’École  de Médecine, 6th arrondissement.  Métro: Odéon.
What a classic pastry shop!  I go out of my way to have breakfast here.  It's also not a bad place for a nice relaxing lunch.  Sit down inside with the Sorbonne students if you wish.  Angelina may have the reputation of having the best hot chocolate in town, but Pâtisserie Viennoise’s is better.  Closed weekends.

Le Pré Verre,  8, rue Thenard, 5th arrondissement.  Métro: Maubert-Mutualite.
You know it has to be good if the Parisians line up to get into the place.  It is near the Cluny, across the street from the Hôtel du College de France.  The French food here is complemented with spices, especially those of the Asian variety.  They offer a prix fixe dinner menu for about 30 euros that makes it the the 2nd best bargain in Paris.  The dining room is also attractive and welcoming, although the seating might be a bit tight.  The best bargain in Paris is their 13.50 euro lunch deal.  You might need to make a reservation a day or two beforehand and make sure you ask to dine in the upstairs room. I enthusiastically recommend this restaurant.  Website:  http://www.lepreverre.com

L’As du Fallafel,  34, rue des Rosiers, 4th arrondissement.  Métro:  St-Paul.
This Jewish deli-type restaurant in the Marais is a real crowd-pleaser.  I like it so much for their 5-7 euro falafels (they call them “Cocktail de Viandes” which literally means “Meat Cocktail) that I keep their business card tacked on my corkboard in my kitchen.  It's not fancy, but it's a great little take-away (“emporter” in French) place or fine for a sit-down lunch.   It is open for dinner, too.   I highly recommend this restaurant.