I'm surprised no one has mentioned grazing. You keep your eyes peeled for little kids with way too much food on their plate and intercept before they hit the trash bin. This is a solid way to get french fries for free. Then you just microwave the can of chili you stashed in the snowbank at the start of the day and voila, you have chili fries.
In places where you put your whole tray, trash and all, on a rack instead of throwing your food away and putting the tray on a stack,(*cough* whistler) then you can just browse the racks for half-eaten anything.
Also, soup crackers are free if you "already bought soup." They go great with butter and honey or peanut butter, all available at the condiment station for free. Or with the brick of cheese and salami you stashed in your pocket.
Ketchup soup is another alternative, but it's kinda gross if you think about it too much.