A convenient User Interface concept, a pie menu is one where the cursor starts in the center of a circle, and menu options surround it. The result can be viewed like a pie chart, with each of n options occupying 1/n of the pie. The user ends up moving the mouse little compared to normal pull-down menus, and power users can easily remember item placement as a function of angles. The Pie Menus concept even allows for submenus: where one pie slice ends, another pie can begin.
To date, The Sims is the most commercially successful application to integrate pie menus. Maxis used the concept extensively in character interactions.