OPie is a radial action-binding addon: it lets you group actions into rings which appear when you hold down a keyboard or mouse binding. When you release the binding, OPie will perform an action based on where your mouse cursor is.
Use OPie to reduce the amount of clutter on your action bars: rings can contain your abilities, items, professions, battle pets, equipment sets, macros, and raid or world markers. Some rings for common class abilities and professions are included, as is a special quest items ring which automatically makes all of your quest and quest-starting items easily accessible. Other addons may add additional rings; for example, Spade uses OPie rings to let you chose the seeds you want to plant on your farm.