In economics, complementary goods are goods where the demand schedules move together. Typically, complementary goods are used together. A classic example is hot dogs and hot dog buns. If the price of hot dogs rises, the demand for dogs falls. So does the demand for buns.
In choice architecture economics, the Nudge blog proposes that complementary goods are also goods frequently used in combination, and goods whose arrangement together can promote better decision making.
Consider the following humorous example at a grocery store. Beer and condoms. Two things that belong side-by-side. (Hat tip: Kelly Carter)
Addendum: Jodi Beggs spots more complementary goods over at the Consumerist. Beer and ping pong balls (for beer pong, of course). The link is below in the comment section, but this photo is too perfect.