For all the vegetarians out there: chickpeas, chickpeas, chickpeas. It's around $1.50 for a can and you can throw them in salads, pitas, wraps, pasta sauces, or vegetable medleys to instantly make a meal much heartier and get an excellent dose of various essential vitamins, protein, and fibre. You can also make them into their own dish (mmm channa) and eat them on their own. Lentils are really tasty and nutritious as well!
Stagg's vegetarian chili is another excellent option. 1 can is around $2.40-2.80 depending where you buy it, and for me makes 2 meals. I'll eat it with whole grain bread/rolls, amounting to maybe $2 a meal.
And, like everyone else has been saying, pasta! Chick peas can be added instead of sausage/beef. I like to throw peppers, broccoli, celery, onion/leek, and carrots into the sauce as well (but I'm kind of a vegetable fiend, and note that fresh vegetables raise the cost of the meal considerably
)
Hash browns are a cheap and easy snack for when you don't feel like cooking. Not all that healthful, but they're super cheap and one pack lasts you a while.