An image of an entire pizza can make a great logo, with an easily-recognizable, circular shape. You can personalize your logo by the toppings that your pizza has on it or by making the image more stylized or more realistic. Viewing the pizza from an angle, making it an oblong, can also be a good idea.
Individual slices of pizza can also work well as a logo. The triangular shape of a slice of pizza is familiar to everyone. Containing multiple slices can add to the creativity of the logo, as can extra cheese dripping off the edge of the slice. Integrating a slice of pizza into the title of your business (say, to replace the letter A) can also be effective.
The range of pizza-related logos possible is nearly endless. Your logo can depict a whole tomato or an onion or, if you're careful not to make it too complex, a pizza shown together with the toppings and ingredients that make it up. A pizza spatula or cutter can also factor into your logo, making it that much better.