Yoshinoya, Matsuya, cheap ramen places, family restaurants, supermarkets when its getting late at night. Filling an apartment with furniture put a real crimp on the budget, so its cheap eatin for me.
Not quite suitable for backpacking but buying things in bulk when on special (say veggies, fruit, meats, whatever), making meals and freezing the excess really helps to save money. This weekend I'm making pizza and I'll probably cook two and save about 3 or 4 for later or parties etc.