No Work Free Pizza
Pizza restaurants are busiest on Thursdays, Fridays and Saturdays, the latter two being the biggest days. What this means is that not only do a lot of people order pizzas, but a lot of the pizzas which are made for people never make it to anybody for one reason or another. Whatever the reason, at the end of the night before closing up shop the employees place anywhere from one or two, and sometimes as many as 15 perfectly good pizzas (still in the box!) in the dumpster behind the store.
These pizzas are always going to be cold, but hell it's free pizza! Many times there are bread sticks there too. The best time to check is about an hour or so after closing time or the next morning. This ensures that the pizza you find is no more than a day old, very important especially in the summer time. Pizza, like all foods, does go bad especially after sitting in the heat long enough.
The best pizza restaurants to check are the big chain ones like Pizza Hut, Domino's or Papa John's. Sometimes, you can ask an employee in the moments before close if they are about to throw away any pizzas and sometimes they'll give you one. But don't count on it. The dumpster is your best bet.
Check out Dumpster Diving Food 101 for more information on other sources of free food.