Free food isn't common online, but one way to get it is to enter a sweepstakes, like those offered by the Food Network, Smart Balance, NatureBox, Old Navy and Pure Wow. These prizes are limited, so they are not a substitute for regular grocery shopping.
Most food sweepstakes focus on specialty items, like chocolate, gourmet popcorn or wine. Some companies offer non-food prizes in addition to grocery store gift cards. Many large grocery store chains offer contests or rewards if you take an online survey after having shopped at their stores. Look at the bottom of your receipt, or ask your cashier to find out if there is a way to enter a drawing for a store gift card.
Coupons for free samples are another online option for obtaining free food, but these are generally offered in small quantities and certainly would not do much to supplement your regular grocery budget unless you spend a significant amount of time searching. Several offers for "free" store gift cards found in a Google search were actually fake websites with links leading to an empty website. Beware of scams that try to get identification information from you or want you to click on a link that spreads a virus to your computer.