As a former pizza jockey:

ALWAYS TIP

Pizza hut had a delivery fee of like $1.25, but only a quarter actually went to the driver. In most cases, they are buying their own gas, paying their own insurance etc, and get payed jack shit (less than minimum wage, in my case).

Also keep in mind if your pizza is late, 90% of the time it's not the driver's fault but rather then store manager routing deliveries. Tipping is a very easy way to improve your karma h: